Good morning GilbertColomy,
It sounds like you’re having some trouble with iMessage, and when sending a message to a contact you’re getting the error that the contact is invalid. I’ll be happy to help out with this.
First, let’s start with some basic troubleshooting. Open Messages > Preferences > Accounts and make sure you’re signed in with your Apple ID here. Also make sure under “You can be reached for messages at” your Apple ID is listed here. If not, then click “Add Email” and add your Apple ID. If these settings are correct, then try signing out of your Apple ID here and then sign back in.
If these settings are correct and you’re still having issues, let us know if it's only this contact that you're having issues with and if you have the same issue sending an iMessage to this same person on your iPhone.
Take care!