The 'match against' should be part of the SELECT clause or the WHERE clause, you have it as part of the JOIN clause
SELECT title, content, author_lname, author_fname, article_url, categor,
match (title, content, author_lname, author_fname, categor) against (' dyslexia ') as relevance
FROM document
WHERE match (title, content, author_lname, author_fname, categor) against ('dyslexia')>0
HAVING relevance > 0
ORDER BY relevance DESC