This Contact doesn't have any associated Messaging Users. Ensure that any Messaging Users have their Contact field set up properly and that they haven't opted out of Messaging

"This record doesn’t have any associated Messaging Users. Ensure that any Messaging Users are set up properly and that they haven’t opted out of Messaging".

1. Make sure the user(Agent) who is trying to start the conversation is added the Queue of the Messaging Channel or have associated Skills if Skill-Based Routing is used.