I followed the maconnolly's guide but when i want to add an existing item in the asset editor, the option is not clickable and i didn't find someone who has the same issues so i hope you guys can help me.
I'm not sure if your starting point was the older version of the template - I am guessing it probably was, as I think it had an incorrect XLP class assigned. The issue you have is because the XLP Class is invalid.
The XLP Class should be: UITexture
This should result in that 'error' no longer appearing in Module Name, but also enable you to add your Textures to the XLP.