I've just set up a account on itch so I could use the app because I can't work out how to open games (certainly not this one, lacking a README.txt or similar), and I tried to log in.  This error was the result. I've closed and opened the app, same result. If it helps I am using Linux Cinnamon 21 (not sure abt the number but def Cinnamon)

This error is telling you the issue. Please double-check that you’re using the correct username and password for your itch.io account. Try logging in from a browser incognito window to ensure that you are using the correct credentials.