Use
On movies.id = seqs.id - 1;

SELECT movies.title, sequel.title as sequel_title
FROM movies
LEFT OUTER JOIN movies sequel
ON movies.sequel_id = sequel.id;

Your thinking is right, but you are not understanding the question.
it is asking to display TITLE of Sequel movie NEXT to the TITLE of the first movie.
Since you wrote "SELECT movies.title, seqs.sequel_id" it will show one column with title in text format and one column with the sequel id in number format. You should therefore change "seqs.sequel_id" to "sequel.title"

SELECT movies.title, seqs.title
FROM movies LEFT OUTER JOIN movies seqs ON movies.sequel_id = seqs.id;

Try this:

SELECT a.title, b.title FROM movies AS a
LEFT OUTER JOIN movies AS b ON a.sequel_id = b.id

This stuff really helps me