There seems to actaully be 4 answers, and just showing the top 3 will work. I don't know if this is intended, but the description of the problem should be changed.

Mybe that's just me but from the way the exercise is worded, I assumed it was expected to write a query that would ONLY insert the new row IF the participant was age 21 or older.
Another way to understand it would be that at any attempt that you try to add a particpant younger than 21 to the table, it would change the 'attending' column to False.

Both ways seem too hard for a Kyu 8 level, but well... the wording should really be reconsidered.

For my fellow strugglers: You need to first simply insert your row and then just print the table with the age-condition.

some of them have already paid tution and some haven't

Easy, but there's two problems: You ask to avoid 'The US', and it should be 'USA' or it will crash. It's silly, but it'll avoid one failed attempt for someone who take it literal.
I couldn't use the 'NOT CONTAINS' , and i think it's a valid answer for this example.

Great Kata!

Maybe in order to learn beginners better practices, you should either include NULL values into the country column of travelers or add a DESCRIBE of the table to the description where we can see the column having a NOT NULL contraint.

I think this would teach good practices to beginners. Feel free to discuss what you think!