I cannot install the modified app

[redatal]

Hello, I've been trying to reuse the app by adding a little modification, I cloned the app and built it after changing this "classpath 'com.android.tools.build:gradle:4.1.1" to this "classpath 'com.android.tools.build:gradle:3.6.1" since I am using Android Studio 3.6.1, the app works fine on the emulator, the problem is that once I push the modified version, pepper downloads, and launches the installation but doesn't install it, and the error message is "app not installed", I thought maybe the problem is in the snippet I added, I delete it and add a comment, the result is the same, the app cannot be installed, I re-clone the app and do not change anything besides the "com.android.tools.build:gradle:4.1.1" it can be installed, is the code protected or is it because of the version of android studio or something else?