It looks like, when you were creating the Meeting Attendance object, you connected it to itself rather than to Meeting (Steps 78-82). Now go back to Setup > Object Manager > Meeting Attendance and Fields and Relationships and then delete the Meeting ID field and then create it again using Steps 78-82 but follow instructions carefully this time. Then Step 99 will work.
It looks like, when you were creating the Meeting Attendance object, you connected it to itself rather than to Meeting (Steps 78-82). Now go back to Setup > Object Manager > Meeting Attendance and Fields and Relationships and then delete the Meeting ID field and then create it again using Steps 78-82 but follow instructions carefully this time. Then Step 99 will work.