Problem with Minecraft PE from MCPE monster.

I tried installing minecraft 1.19 (beta version) from MCPE monster. I have been downloading from mcpe monster since 1.16 or 1.15, had no problems except, the APK failed to install, it used to get fixed after signing the apk.

But now, signing doesn't work. Installer fails with error "App not installed".

I am trying to install it on android 6. Maybe that could be the problem, but MCPE in playstore is available for purchase (means that compatibility isn't the issue).