Apk extractor from F-Droid doesn’t seem to work. App in question is Typewise Offline (android keyboard with hex-shaped keys).
EDIT: Probably worth a separate post at some point, but this is one of the few apps for which there is no open source equivalent, TTBOMK, and one of the few remaining proprietary apps that I rely on.
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.