Yes that is a storage issue. XAPK is a combined file format consisting of the .apk installation package and the OBB data file. I think your OBB is being placed incorrectly. When the game loads up it is looking for the OBB but does not have the permissions to find it. Your app doesn’t ask for permissions and hence you get the Validation Failed Error message. Make sure you have checked the use ExternalFilesDir in your project settings and that should hopefully solve the the validation error or give you a message asking for storage permission.
Still quite strange, since you manually turned on the storage permissions, you shouldn’t be running into the validation issue. Can you check if your OBB file is in your Android/obb folder?