Fix There has been an error joining a session in GTA Online
GTA Online - There has been an error joining a session. Please return to the Main Menu.
It’s not you — the issue is on their end. Their online services can experience a problem, which usually happens when the Rockstar game servers are down.
You'll need to wait until the servers are restored which usually take a like few hours
While you wait you can try the steps below
Restart your device & Router and relaunch your game again
Try to verify your game files
Try deleting this files
Documents\Rockstar Games\Social Club
Documents\Rockstar Games\Launcher
C:\Program Files\Rockstar Games \Launcher
C:\Program Files\Rockstar Games\Social Club
C:\Program Files (x86)\Rockstar Games\Social Club
After this Download the latest Rockstar Games Launcher your Launcher will: Reinstall Social Club automatically and fix the error joining session