Can't Change Keybinds

I can clear binds with RMB as instructed but it wont let me input a new one.

It will let me input LMB as the new bind (it lets me know i'm changing it from attack) but that's it! Keyboard works fine what's the problem here.

I've been at this over an hour, pretty infuriating

Forgive me if im missing something simple here and thanks looking at my post.