This means you don't have enough RAM left. Either it'll not work on your device at all or you just have too much RAM in use.
Either your browser doesn't work with This tool or you don't have enough RAM left
This means that you don't have enough RAM left. Try closing all other tabs and Apps. If the problem still occurs (and you have RAM left) Then the patch is too big to be used. You can see estimated RAM usage above.
This means you have a version of your app installed which is newer than the one you want to install. Simply uninstall your app before trying to install the downgraded apk (Note: without creating an Game Data Backup in the apps section of Sidequest first you may loose game progress)
First make sure you haven't tried to mod your game. If this is the case uninstall Beat Saber on your Quest and download it again. Then make a new backup and try again. Otherwise just try backing up the APK again.
Make sure you haven't checked skip verification as this will skip any checks of your APK, Patch and downgraded APK. This error usually means the downgraded APK is corrupted.
Usually this means that your browser doesn't work with the downgrader (so try some different browsers)
This error usually occurs on opera and indicates that your device has too little ram available