What can I do about a jammed tub drain plug?
There may just be a bit of gravel or something wedged in the mechanism. Water will drain, but the mechanism doesn't work. Ensure the area is fully clear. If the linkage has failed outside of the drain, it may require removal of finish material either below or behind the tub drain to access and repair/replace the mechanism. If you're lucky, there's a removable panel somewhere.

It may be easier remove the current stopper and use an old fashioned rubber plug. If the current stopper isn't easily removed, and it's of a low enough profile, a 'universal' stopper may work. It's just a flat piece of rubber that seals against the tub floor. This will also work if there's some sort of fixed strainer preventing the use a conventional plug.