No signature of method: build_

I've been looking all over GitHub, Stack Overflow, Reddit, Discord servers, everything. I've recently implemented firebase into a project I'm doing for a Udemy course (Flash Chat app).

Obviously the work they're doing is outdated, so I've been looking around to find the latest versions and the lot. Though when everything is imported and should work, I get this error:

No signature of method: build_eeszr0gcr6lj33p64zor43zf6.android() is applicable for argument types: (build_eeszr0gcr6lj33p64zor43zf6$_run_closure2) values: [build_eeszr0gcr6lj33p64zor43zf6$_run_closure2@35f07b5d]

All of the forums I'm looking at tell me to remove experimental true somewhere, but nowhere can I find this in my build.gradle files.

Here are the "relevant" files:

android/app/build.gradle

def localProperties = new Properties()
def localPropertiesFile = rootProject.file('local.properties')
if (localPropertiesFile.exists()) {
    localPropertiesFile.withReader('UTF-8') { reader ->
    localProperties.load(reader)
    }
}

def flutterRoot = localProperties.getProperty('flutter.sdk')
if (flutterRoot == null) {
    throw new FileNotFoundException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
}

def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
if (flutterVersionCode == null) {
    flutterVersionCode = '1'
}

def flutterVersionName = localProperties.getProperty('flutter.versionName')
if (flutterVersionName == null) {
    flutterVersionName = '1.0'
}

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
apply plugin: 'com.google.gms.google-services'
    
android {
    compileSdkVersion 30

    sourceSets {
        main.java.srcDirs += 'src/main/kotlin'
    }  

    defaultConfig {
        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
        applicationId "com.DaemonDev.application"
        minSdkVersion 16
        targetSdkVersion 30
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
        multiDexEnables true
    }

    buildTypes {
        release {
            // TODO: Add your own signing config for the release build.
            // Signing with the debug keys for now, so `flutter run --release` works.
            signingConfig signingConfigs.debug
        }
    } 
}
    
flutter {
    source '../..'
}

 dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation platform('com.google.firebase:firebase-bom:28.3.1')
     implementation 'com.google.firebase:firebase-auth'
    implementation 'com.google.firebase:firebase-firestore'
    implementation 'com.google.firebase:firebase-storage'
    implementation "com.android.support:multidex:$multidex_version"
}

android/build.gradle

buildscript {
    ext.kotlin_version = '1.3.50'
    repositories {
        google()
        jcenter()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:4.1.0'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath 'com.google.gms:google-services:4.3.10'
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

rootProject.buildDir = '../build'
subprojects {
    project.buildDir = "${rootProject.buildDir}/${project.name}"
    project.evaluationDependsOn(':app')
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

gradle.properties

org.gradle.jvmargs=-Xmx1536M
android.useAndroidX=true
android.enableJetifier=true
android.enableR8=true
android.enableDexingArtifactTransform=false

local.properties

sdk.dir=C:\\Users\\Usr\\AppData\\Local\\Android\\sdk
flutter.sdk=C:\\flutter
flutter.buildMode=debug
flutter.versionName=1.0.0
flutter.versionCode=1

android/gradle/gradle-wrapper.properties

distributionBase=GRADLE_USER_HOME
distributionPath=wrapper/dists
zipStoreBase=GRADLE_USER_HOME
zipStorePath=wrapper/dists
distributionUrl=https\://services.gradle.org/distributions/gradle-6.7.1-all.zip

I've imported the Firebase core, auth and cloud-firestore only.

All the related issues have resolutions that seem not applicable. What can I try to fix this?