I won’t be able to help you here (since I have no Blender experience). I however would suggest going to forums dedicated to Blender (or joining relevant servers on Discord), as it seems Rōblox’s idiosyncrasies don’t affect what you’re trying to do.
What are you trying to achieve; if you are attempting to join the vertices, you can press alt + m while selecting both vertices and click “merge vertices”. If that is not what you are looking for, then please elaborate.