That's not valid syntax, which engine is running it?
You either need:
SELECT title, max(postid) FROM post GROUP BY title;
to get multiple records, one for each title, showing the max postid for each title, or
SELECT max(postid) FROM post
to get the single max postid from the table.
If you want the highest postid and the title that goes with it, you need
SELECT TOP 1 title, postid FROM post ORDER BY postid DESC
or
SELECT title, postid FROM post ORDER BY postid DESC LIMIT 1
depending on your SQL engine.