How to get rid of this index out of range error? I know the reason but don't know the solution

All problem is that you open other page (Get an Offer) and after going back all selections are removed - browser expects that user will start from the beginning but code expects that some sections will be still selected.

It may need to remember all selections and later click them again.

Here I remeber selections in item_index

item_index = [0, 0, 0, 0, 0, 0]  # remember all selections

for i in range(len(processor)):
    item_index[0] = i  # remember selection
    # ... code ...

    for g in range(len(generation)):
        item_index[1] = g  # remember selection
        # ... code ...

        for r in range(len(ram)):
            item_index[2] = r  # remember selection
            # ... code ...

# ... etc. ...  

And later select them again

    driver.execute_script("window.history.go(-1)")

    for number, index in enumerate(item_index):
        print(f'[reselect] section {number} -> index {index}')
        sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
        items = sections[number].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")
        driver.execute_script("arguments[0].click()", items[index])
        time.sleep(3)

Full working code:

from selenium import webdriver
from selenium.webdriver.chrome.service import Service
from selenium.webdriver.common.by import By
from webdriver_manager.chrome import ChromeDriverManager

import pandas as pd

import time
import random
import string

# --- functions ---

def random_char(number):
    """For generating fake email."""
    return ''.join(random.choices(string.ascii_letters, k=number))


def reselect(item_index, sleep_time=3):
    for number, index in enumerate(item_index):
        print(f'[reselect] section {number} -> index {index}')
        sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
        items = sections[number].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")
        driver.execute_script("arguments[0].click()", items[index])
        time.sleep(sleep_time)

# --- main ---

driver = webdriver.Chrome(service=Service(ChromeDriverManager().install()))

search_url = "https://northladder.com/en/ae/electronics/laptop/lenovo/thinkpad/v1/5e2c888401c848ac4f695bb8"
driver.get(search_url)
time.sleep(3)

ALL_ITEMS = []
item = ["", "", "", "", "", "", "", "", "", "", ""]
item_index = [0, 0, 0, 0, 0, 0]  # remember all selections

sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
processor = sections[0].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")

for i in range(len(processor)):
    print('sections[0]:', i)

    sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
    processor = sections[0].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")
    item[0] = processor[i].text
    item_index[0] = i  # remember selection

    #processor[i].click()
    driver.execute_script("arguments[0].click()", processor[i])
    time.sleep(2)

    sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
    generation = sections[1].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")

    for g in range(len(generation)):
        print('sections[1]:', g)

        sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
        generation = sections[1].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")
        item[1] = generation[g].text
        item_index[1] = g  # remember selection

        #generation[g].click()
        driver.execute_script("arguments[0].click()", generation[g])
        time.sleep(2)

        sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
        ram = sections[2].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")

        for r in range(len(ram)):
            print('sections[2]:', r)

            sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
            ram = sections[2].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")
            item[2] = ram[r].text
            item_index[2] = r  # remember selection

            #ram[r].click()
            driver.execute_script("arguments[0].click()", ram[r])
            time.sleep(2)

            sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
            model = sections[3].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")

            for m in range(len(model)):
                print('sections[3]:', r)

                sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
                model = sections[3].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")
                item[3] = model[m].text
                item_index[3] = m  # remember selection

                #model[m].click()
                driver.execute_script("arguments[0].click()", model[m])
                time.sleep(2)

                sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
                screensize = sections[4].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")

                for s in range(len(screensize)):
                    print('sections[4]:', s)

                    sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
                    screensize = sections[4].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")
                    item[4] = screensize[s].text
                    item_index[4] = s  # remember selection

                    #screensize[s].click()
                    driver.execute_script("arguments[0].click()", screensize[s])
                    time.sleep(2)

                    sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
                    drive = sections[5].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")

                    for d in range(len(drive)):
                        print('sections[5]:', d)

                        sections = driver.find_elements(By.XPATH, '//div[@class="select-common-wrap"]')
                        drive = sections[5].find_elements(By.XPATH, ".//label/div[@class='select-item m-1']")
                        item[5] = drive[d].text
                        item_index[5] = d  # remember selection

                        # you forgot it
                        sections = driver.find_elements(By.XPATH, '//div[@class="ant-form-item-control-input-content"]')
                        driver.execute_script("arguments[0].click()", drive[d])

                        print('mail')
                        email = random_char(7)+"@gmail.com"
                        email_input = sections[0].find_element(by=By.ID, value='checkout-user-detail_email')
                        email_input.send_keys(email)
                        time.sleep(1)

                        print('click button')
                        button = driver.find_element(By.XPATH, '//button[@class="ant-btn ant-btn-primary lock-price primary-btn"]')
                        driver.execute_script("arguments[0].click()", button)
                        time.sleep(5)

                        print('prices')
                        quote = driver.find_element(By.XPATH, '//div[@class="m-auto section-wrap"]/ul[@class="four-grade-pricing-wrapper"]')
                        price_quote = quote.find_elements(By.XPATH, ".//li[@class='fgp-item']/h1[@class='price']")
                        for number in range(5):
                            item[6+number] = price_quote[number].text
                            print(f'{number} -> {price_quote[number].text}')

                        print(item)
                        ALL_ITEMS.append(item.copy())  # duplicate `item` because I will use the same list to get new results

                        print('go back')
                        driver.execute_script("window.history.go(-1)")
                        time.sleep(3)

                        reselect(item_index)

for item in ALL_ITEMS:
    print(item)

my_df = pd.DataFrame(ALL_ITEMS)
my_df.to_csv('lenovo_thinkpad.csv', index=False, header=False)