IMessages & ICloud

That message indicates that you are using a different Apple ID for Messages than you are using for iCloud. See the following previous discussion response from an Apple Community Specialist: iCloud and iMessage Accounts are different - Apple Community

If the message is correct, you are using 2 Apple IDs. You can see what they are.

See the following from the link I previously referenced iCloud and iMessage Accounts are different - Apple Community

On your iOS device, go to Settings > Your Name and see what the Apple ID is at the top of the screen. From there, go to Settings > Messages > Send & Receive and see what Apple ID is signed in.

Is your Apple ID shown 3 times on that screen -- once in "You can receive iMessages to and reply from", once in "Start new conversation from" and once in very small print below that?