If Facebook login requires you to enter code from Code Generator when it has not registered the device, app or browser as something you've used before for Facebook login.
Start the Facebook app, tap Menu - Settings and privacy - Code Generator. The tap the shown code to copy it into clipboard. Go back to Grosh and long press in the code field to paste the value, then continue the login and you should come into the Grosh app.
Note: If you can’t find the Code Generator in the Facebook app, try to relaunch the Facebook app. There seems to be a bug so that it does not appear every time.