Unable to remove push-pull stopper from tub drain
You just have to grip the whole top assembly with a channel lock or vise grip, taking precaution to not damage the finish. Slowly turn counter-clockwise. Also trying squirting WD-40 with the tube under the cap, Wait a few minutes, and try again.  Removing the cap off the stem will reveal the pop-up mechanism.  I have a picture of my cap removed.  Either the cap will come off at the top or the whole stem will unscrew at the "Cross" about 1.5" down in the drain. The stem in the middle of the drain will unscrew from the cross bar you see even if the cap is stuck. Using WD-40 will either loosen the bottom or top or both. When using the Pliers, use a rag to protect the edge and give gentle twist counter-clockwise.



Also try this using a small wrench,  The size that slip on the flat sides on mine was 10MM,  about 3/8".  In the tub, it should be flat enough to just slip the wrench in.


IF the whole assembly is rotating, see picture below, then the retaining nut beneath the tub has loosened.  Remove the retaining nut, disconnect drain from downspout, and remove the complete assembly.