Here is a solution for your problem! :-)



First uninstall your old XAPK package installer


After that

1. Enable unknown sources for your browser in Android Settings.

Go to Settings > Biometrics and security > Install unknown apps

2. Then tap your Internet Browser and Enable: 'Allow from this source'

3. Close the window


Then download UpToDown app from their site


After that

1. Find the file in the download section of your browser
2. Tap the file to install the app (don't run)

3. Then again go to Settings > Biometrics and security > Install unknown apps

Tap the app UpToDown and enable: 'Allow from this source'

Close the window

4. Download your XAPK file from this site
5. Run the XAPK file from your browser download section


Now the app is being installed by the UpToDown package installer which should work fine.