If your device is below min version of 21 SDK - Android 5 - it won't install - any users who have devices below that - and there still are users out there with those devices, won't be able to update - the app won't install.
I'm not sure where you're publishing this but Google Play won't accept app updates on 28 SDK - only on 29 SDK - i.e. Android 10. In other words, they'll reject your update and there will be nothing to update for the users.
If you're doing a direct install then you could have a block on Google Play Protect. Make sure that is disabled. They block apps that don't come from the Google Play Store.