That means either the key bind is empty, or that you remapped the key bind.
Go into options, key binding, and map the keys to your choosing.
I have gotten this pop-up as well while playing the game, but since I manually remmaped the keys, I knew what I needed to press.
All hot keys are supposed to be mapped with a default value and if you did remap anything it's supposed to automatically bring up the remapped key when it comes on the screen like every other game I've played.
The bottom line is that it's unexcusable for the game to be in this condition, especially with no instructions on how to fix it.
