iCloud Signin is asking for an old device passcode

Possibly sounds like iCloud keychain, it will asks for the passcode of an old device or ask to be approved on one of your other devices. There should be an option to reset or don't have access, it may be on the screen before ti asks for the passcode.

The code should be sent to all of your trusted devices. It can also be sent to your trusted telephone number. That code is not a device passcode. It has nothing to do with any device/hardware you own as it is designed to protect your Apple online services ID (AplleID). It is a 6 digit code sent by notification and which expires after a few minutes. It is a temporary, dynamically generated code sent by the 2FA system every time you login to your AppleID or use your AppleID with a new device.

Two-factor authentication for Apple ID - Apple Support

You would need a 2 factor authentication code to sign into your AppleID so how did you get that code when you signed into your AppleID account?

If you don’t have any available trusted devices, or never set up a backup trusted telephone number, you will have to call Apple for help.

Signing into iCloud won’t ask for the screen lock passcode. It will require the password for the AppleID you use to sign in to iCloud. You cannot have an AppleID without assigning it a passcode.