I pressed Fulfill  but nothing is happening

Make sure you have updated the inventory to accommodate the waitlist orders. Make sure to Publish that inventory update (not just Save as Draft).

I Fulfilled the Waitlist request, but I don't see it in Orders

Go to the Search bar in Orders and type "Declined". If a Waitlist card did not process it won't show in Orders.

I turned Waitlist OFF, but I still see it

All Waitlist requests must be fulfilled, expired, or canceled for Waitlist to fully turn off. If you have fulfilled all waitlist requests and added new inventory for sale and your waitlist is still visible, check your "Lock Waitlist Once Started' settings and ensure it is OFF.