cstack on "Insert user_id"

I'm trying to INSERT the current_user into a separate db for future querying. I want it to insert into the most recent entry - ie. INSERT_LAST_ID().. but it's not working with an INSERT query using WHERE=INSERT_LAST_ID()
or with an UPDATE query using same. I've also tried MAX(id) in place of it with no luck. I can, however, successfully query SELECT MAX(id) FROM table1.

Anyone know why this max id doesn't want to work?
Thank you