Selenium/BeautifulSoup - WebScrape this field

Your problem is with driver.find_elements_by_class_name('item-expand-action expand') command. The find_elements_by_class_name('item-expand-action expand') locator is wrong. Those web elements have multiple class names. To locate these elements you can use css_selector or XPath.
Also since there are several elements with dropdowns, to perform clicks on them you should iterate over them. You can not perform .click() on a list of web elements.
So your code should be like this:

ifDropdown=driver.find_elements_by_css_selector('.item-expand-action.expand')
for drop_down in ifDropdown:
    drop_down.click()
    time.sleep(0.5)

Alternatively to the css_selector above you can use XPath as well:

ifDropdown=driver.find_elements_by_xpath('//a[@class="item-expand-action expand"]')

UPD
If you wish to print the added, new titles you can do this:

ifDropdown=driver.find_elements_by_css_selector('.item-expand-action.expand')
for drop_down in ifDropdown:
    drop_down.click()
    time.sleep(0.5)
newTitles=driver.find_elements_by_class_name('card-title')
for new_title in newTitles:
    print(new_title.text)

Here after expanding all the dropdown elements I'm getting all the new titles and then iterate over that list printing each element text.
driver.find_elements_by_class_name returns a list of web elements. You can not apply .text on a list, you have to iterate over list elements getting each single element text each time.
UPD2
The entire code opening dropdowns and printing their inner titles can be like this:
I'm doing this with Selenium, not mixing with bs4.

from selenium import webdriver
from bs4 import BeautifulSoup
from selenium.webdriver.common.action_chains import ActionChains

import time
driver = webdriver.Chrome()
actions = ActionChains(driver)

driver.get('https://cslide.ctimeetingtech.com/esmo2021/attendee/confcal/session/list')
time.sleep(4)
page_source = driver.page_source
soup = BeautifulSoup(page_source,'html.parser')

new_titles = set()

productlist=driver.find_elements_by_xpath("//div[@class='card item-container session']")
for property in productlist:
    actions.move_to_element(property).perform()
    time.sleep(0.5)
    sessiontitle=property.find_element_by_xpath(".//h4[@class='session-title card-title']").text
    print(sessiontitle)
    ifDropdown=property.find_elements_by_xpath(".//*[@class='item-expand-action expand']")
    if(ifDropdown):
        ifDropdown[0].click()
        time.sleep(4)
        open_titles = driver.find_elements_by_class_name('card-title')
        for open_title in open_titles:
            title = open_title.text
            if(title not in new_titles):
                print(title)
                new_titles.add(title)

Here I am checking if there is a dropdown. If do, I open it. Then getting all the currently existing opened titles. Per each such title I validate if it is new or was opened previously. If the title is new, not existing in the set I print it and add it to the set.

To get all the data, including the date, time, chairs, you can use only requests/BeautifulSoup. There's no need for Selenium.

import requests
import pandas as pd
from bs4 import BeautifulSoup


data = []
url = "https://cslide.ctimeetingtech.com/esmo2021/attendee/confcal/session/list?p={}"

for page in range(1, 5):  # <-- Increase number of pages here
    with requests.Session() as session:
        soup = BeautifulSoup(session.get(url.format(page)).content, "html.parser")
        for card in soup.select("div.card-block"):
            title = card.find(class_="session-title card-title").get_text()
            date = card.select_one(".internal_date div.property").get_text(strip=True)
            time = card.select_one(".internal_time div.property").get_text()
            try:
                chairs = card.select_one(".persons").get_text(strip=True)
            except AttributeError:
                chairs = "N/A"

            data.append({"title": title, "date": date, "time": time, "chairs": chairs})

df = pd.DataFrame(data)
print(df.to_string())

Output (truncated):

                                                                                                                                                                         title             date           time                                                                    chairs
0                                                                                                                                                Educational sessions on-demand  Thu, 16.09.2021  08:30 - 09:40                                                                       N/A
1                                                                                                                                                    Special Symposia on-demand  Thu, 16.09.2021  12:30 - 13:40                                                                       N/A
2                                                                                                                                          Multidisciplinary sessions on-demand  Thu, 16.09.2021  16:30 - 17:40                                                                       N/A
3                                                                                                                    MSD - Homologous Recombination Deficiency: BRCA and beyond  Fri, 17.09.2021  08:45 - 09:55                       Frederique Penault-Llorca(Clermont-Ferrand, France)
4                                                                                                          Servier - The clinical value of IDH inhibition in cholangiocarcinoma  Fri, 17.09.2021  08:45 - 10:15  Arndt Vogel(Hannover, Germany)Angela Lamarca(Manchester, United Kingdom)
5                                                                                                                   AstraZeneca - Redefining Breast Cancer – Biology to Therapy  Fri, 17.09.2021  08:45 - 10:15                                Ian Krop(Boston, United States of America)