Yeah, in Steps 78-82 you were supposed to connect the Meeting Attendance object to the Meeting object but you connected it to itself instead. Now to fix, just go back to Setup > Object Manager > Meeting Attendance and delete the current Meeting ID field and then add the new correct one using those steps (78-82) so it connects to Meeting. Then skip back down to Step 104 and add the new formula field and when you get to 110, it will work.