Might be a split '.apk' file (aka '.apks'),
for which you can try SAI (Split APKs Installer, to backup + restore):
https://f-droid.org/packages/com.aefyr.sai.fdroid/
Might be due to a check to see if the app was installed from the PlayStore,
for which you can try KingInstaller (Spoofs as PlayStore, does not work for split APKs, to restore):
https://apt.izzysoft.de/fdroid/index/apk/com.example.kinginstaller
Might be due to custom licensing check,
then you'd need to decompile, reverse engineer and write a bypass.