I've been trying to bind push to talk to 2 keys so I could record my mic for when I talk in Discord and/or in-game but I've had no luck. If this isn't a feature, then I in my opinion it might help for when someone needs, or wants to talk to others on the server and not make it seem like a one sided conversation
We've found a bug where if (for instance) 'Mute Mic' in Open Talk is already bound to ` and then you try to assign the Push-to-Talk key as ` it will give the error that the key is already in use.
We'll be looking into fixing this in the future, but this sounds like it might be what you're coming across.
Unless you are trying to use Ctrl, Shift or Alt. We're aware those keys currently don't work.