Your subquery needs to return all of the columns needed for the main query.
The main query returns the creator and lessontitle columns. your subquery is only returning the lessontitle column, since the subquery is the dataset that the main query is pulling from, it is generating the error because the creator column does not exist.
When returning more than column, the GROUP BY command is ussed to create distinct results. Try using the following query:
SELECT creator AS XLABEL, COUNT(lessontitle) AS YVALUE FROM (SELECT creator, lessontitle FROM lomaster GROUP BY lessontitle) GROUP BY creator