Update android objectbox from 2.9.1 to newer

Question

I am trying to update android objectbox from V2.3 to newer one. The newest would be V3.5.0. But I can't, the MyObjectBox class is not found.

In the latest version, in the documentation, the warning is written:

Restore compatibility for Android projects using Gradle 6.1. The minimum supported version for Gradle is 6.1 and for the Android Gradle Plugin 3.4. This should make it easier for older projects to update to the latest version of ObjectBox.

I did that, the gradle is 6.1 and the plugin 3.4.0.

Trying to go back, from 3.5.0 backwards, I got it in version 2.9.1. That is, if I put version 2.9.1 it works perfectly. Any version above 2.9.1 the non-ObjectBox class is not found.

Can anyone help me find the problem?

My gradle looks like this:

buildscript {

    ext.objectboxVersion = '3.5.0'

    repositories {
        mavenCentral()
        jcenter()
        google()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.4.0'
        classpath 'com.google.gms:google-services:4.3.14'
        classpath "io.objectbox:objectbox-gradle-plugin:$objectboxVersion"}
}

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

And my app checks are:

compileSdkVersion 31

    defaultConfig {
        applicationId "com.br.mpragueiro"
        minSdkVersion 24
        targetSdkVersion 31
        versionCode 414
        versionName "7.2.1"
        multiDexEnabled true
    }