Hey there,
Sorry for the bad experience.
If you see a red box while reassigning keys, it means the selected key has already been assigned to another function (such as the Space Crisis mode). Please scroll down the Control settings screen to check if any other keys are highlighted in red.
If so, try reassigning that function to a different button, and then proceed to assign the keys you wish to change.
We recommend assigning the function to a key not already in use.
Thanks for your understanding and support.
Anomunus