Until a permanent solution is found
If you own a digital copy of Mortal Kombat Xon PS4, you may have encountered a nasty error that goes by the name ofCE-34878-0. In short, it screws with online play, and for the time being you’ll have to do a little Texas two-step around it until developer NetherRealm can provide a full fix.
What you’ll want to do is sign out of the PSN manually outside of Mortal Kombat X, restart it, and sign back in after the menu appears. Then you’ll be able to play online, as well as be advertised to for all of the hot new DLC that didn’t come with your launch copy.
Error CE-34878-0 [WB Games Support]