Restoring corrupted bak file
you can get this message in different error/ issues on during the backup/restore process. To identify and fix this problem. 


You might using different backup version than what your current database. This is the most probable cause of what I suspect from the error message displayed. You might have a backup file from higher version and now your current server is lower version. you can always restore from lower to higher but can not backward. 
The second issue could be, you might have a corrupted backup file. So first make sure you have restore able backup file. 
Your backup file might be encrypted. Make sure your backup is not encrypted and if so you might need to find the decryption key.


Hopefully this will resolve your issue.