It appears that j by itself has not been marked as a legal character. You must uncheck Character combinations and check j by itself as legal, and the error will go away. Another option would be to add j/J to the alphabetical characters list and then single j will be automatically marked as valid.
The question then becomes, what about j by itself? shouldn’t that be marked as illegal? Yes, it can be, but you must do the work in the Character check Character combinations to mark an uncombined j as invalid.
By the way, if you add j/J to the alphabetical characters list then you will not be able to mark an uncombined j as invalid in the Character check Character combinations inventory. So the best option is to simply check j by itself as valid in the Character check inventory with Character combinations unchecked.