Most likely, the whole element is screwed in instead of the screw (shown by the arrow).
To unscrew it, you need a special key, which was included in the set. And it would be good to unscrew it and sand the unevenness on the bottom of the bathtub with fine sandpaper (where the seal is). You can also try to apply a thin layer of sanitary silicone to the gasket before installation. If you do not have this key, you can try to insert, for example, pliers as open as possible from the top and gently try to unscrew it. Don't use a lot of force, because in this case you will only act on two arms (and the key on four).
