Are you on chrome? I don't think you even need to be logged in to download normally. Maybe try downloading again later or from a different connection, or downloading through the itch.io app: https://itch.io/app
"Forbidden" or "Failed - Forbidden"
This error means you don't have permission to download this file from the server.
To fix, go to the website where the file is hosted. Check if you need to sign in (or provide some other authentication). If you can't sign in, contact the website or server owner, or try finding the file on a different site.