Basic set up issues

[ontap]

Sorry for posting such basic questions.

I am having issues setting up the "back" key  to use on a windows PC . Using the controller set to kodi or full keyboard, when i click on the back key on flirc app then any key on remote , I cannot get it to accept me the key on the universal remote I am setting up. Key just continues to flash green. Also I have a button on the remote that if i press on it to try and set it as a key , it says "button already exists" , I can't remember which key i originally set for this button and when I select "Redo" then try again it still says "button already exists". ?