How to remove point in Blender?

Question

Blender

I don't know what I did wrong during the creation of the figure, but now one point on the character's leg is circled in red dotted line. That's okay, but I can't rotate it or delete it. Rather, when it is removed, the entire model breaks, although everything is fine with the rest. In addition, the surface at this point is black. I would like to get rid of this point (recreate it), but how?