[2021] Can not connect to a friend's Stellaris Game (Cross-platform) after Patch 2.8.1.2

Your friend will have to opt into the stellaris_beta version, in the Steam Betas tab for this game. That uses Paradox MP services instead of Steam's.  

Since you are not on Steam you use a version of this game that uses Paradox MP services, not Steam's . Your friend on Steam by default uses Steam's MP services, but if they opt into stellaris_beta they will then be in the Paradox MP universe and you'll be able to play each other.  

Jadenbetter said: Me and my friend are trying to do this but we have tried both cross play betas and have had an error that the host is playing a different version both times and the other betas are for previous versions of the game anyway

You must use exactly the stellaris_beta version, NOT the "crossplay" or any other beta there.

If you use that beta but have different versions or checksums, there's some other problem here.  

My apologies, it's stellaris_test  

Right click on the game in the Steam library, properties, betas. Click on the drop list there, stellaris_test is the last one listed.  

That version uses a different stellaris.exe which will need to be let through all firewalls separately to the standard one.