As stated in the error message, you have to use an iCloud email address that is already on file with your Apple ID account. An iCloud address that isn't on file with your account is a separate Apple ID. You can't merge Apple IDs.
Based on what I have read, I suggest you leave your Apple ID email address as a 3rd party email address because you have more flexibility if you need to change the email address. For example, if the Apple ID got compromised, you can change it to another address. I also have read of many people who were locked out of their accounts when they forgot their password. Since the recovery email address is the @iCloud email address, if you forget your password, you also can't access your email to get emails from Apple to recover your account. You can set a rescue email address, but for some people, this hasn't worked. You can get another email address from AOL, Yahoo, or other places.