Remove Tool leaving a red mark instead of removing the object when painting the area.

The red mark could be due to working on a smart object, so ensure your layer is rasterized. Also, check that the tool’s opacity is set to 100% and the blend mode is "Normal." Expanding the selection area may help improve removal accuracy. Make sure Photoshop is updated to the latest version, as bugs are often fixed in updates. If the issue persists, try using Edit > Content-Aware Fill as an alternative. Lastly, resetting Photoshop preferences (Ctrl + Alt + Shift on launch) can resolve unexpected tool behavior.

There was a very similar issue in this thread.  After much hair pulling and frustration, and several posts to the forum, the issue appeared to fix itself after restating Photoshop a couple of times.  Also try restarting your computer perhaps.