What is wrong here?


I wrote: select reviewer_name from ( select reviewer_name,number_of_ratings from
reviewers a inner join ratings b on a.reviewer_id=b.reviewer_id) x where number_of_ratings is null


you are checking for number_of_ratings but you should check for reviewer_stars and the join can be made a bit nice. Something like :
SELECT rev.reviewer_name from ratings as rat join reviewers as rev on rat.reviewer_id = rev.reviewer_id where rat.reviewer_stars IS NULL;