My packaged game Used the NULL OSS. I tried destroying the session and a few other things. Latter durring packaging I noticed this log, “LogOnline: OSS: Unable to create OnlineSubsystem instance Steam”.
If you get the same log, just ignore it, lol. It’s a rabbit hole.
FIX:
When it finishes packaging open the folder: CoOpAdventure\Binaries\Win64\ Create a new text file and call it: steam_appid.txt
Open the file and write 480. Save it and start your game. It should work now
The second machine has to use a different Steam login ( I used my wife’s).