Hi Kluppelbergs,
If we understand, you're seeing an alert to sign in to an iCloud account that you no longer have. We want to help. To remove that account, you will need to enter the correct Apple ID account details. Did you delete the Apple ID by requesting it through Apple, or do you just no longer have access to the email address the account is linked to? If you deleted the Apple ID from Apple, you may not be able to sign out of that account. More details about that can be found here: How to delete your Apple ID account
If you didn't delete the account, but no longer have access to it, have you tried resetting the password to the account by following these steps: If you forgot your Apple ID password
If you're able to regain access to the account, you should be able to remove it from your computer. We hope this helps.
Take care!