Error "package android.support.v7.app does not exist"

If the issue reported from MainActivity.java then replace

import android.support.v7.app.AppCompatActivity;

with

import androidx.appcompat.app.AppCompatActivity;

First of all check if your project is using androidx or android support library. Check gradle.properties file:

android.useAndroidX=true

android.enableJetifier=true

If it contains the above lines, it is using androidx with an old code from some old tutorial.

In build.gradle (module:app)

Use

implementation 'androidx.appcompat:appcompat:1.0.0'

Instead of

compile 'com.android.support:appcompat-v7:28.0.0'

Also in MainActivity.java : Use

import androidx.appcompat.app.AppCompatActivity;

instead of :

import android.support.v7.app.AppCompatActivity;