I'm working on Blender Guru's anvil tutorial. At this point he insets a face and uses the boarder option to achieve the desired result. I'm using v2.9 which no longer supports the B shortcut, but I can enable it in the new sub-menu for the command. Nevertheless, nothing happens and the boarder piece remains, so I can't perform the extrude on a single entity; it comes out as two spikes. What am I doing wrong?
I actually encountered the same issue, and wasn't even working after trying @emir process (on the answer). The only way I found was to move the inset face until it snapped the central edge. I haven't finished the anvil yet so i don't know if it will cause me problems, but for the moment it's OK
I figured it out: delete the hidden face as suggested above by Emir. but then also delete this edge. the reconnect the two vertices to create the same edge and then try the inset. it worked for me and I hope this helps. or just turn on clipping in the mirror modifier ( I just found that one out.