Unable to use M1 as push-to-talk

Hello!
I used to play Overwatch 1 of course, and my keybindings were very odd: I used to have my push-to-talk on m1 and my primary fire on a thumb button on my mouse.

Either way, now the game won’t allow for the m1 button to be bound to push to talk, it would be lovely to revert that and make it possible again!

Have a good day~