Can not join game session because the host is running a different version than you

Trying to play with my friend who also owns AOE2 DE but he is getting that message when he accepts my invite.

Each of you needs to make sure you’re running the latest version. There was a patch released yesterday.

If you purchased via Steam, you need to update it via Steam.

If you purchased via Microsoft Store, the update is not yet available. There is a workaround near the bottom of this thread for Microsoft Store users: Patch Not Available in Windows Store