Welcome to AC forums. You've provided some decent info in your post, thank you, but left out some as well.
Does this only happen with this one contact? If it seems to only be happening with this one contact, the first question is what is different about this contact then others you are successfully communicating with? Is their number listed as a landline in their contact information? Some apps won't attempt to send messages to numbers that aren't listed as mobile numbers, they usually offer a better or more specific error message but that leads me to my next question...
What is the messaging app that is causing you issues? If it is an app that you and the other person must both have and they don't have it that could account for the "invalid contact" error. Many messaging apps want to connect to your contacts, so seeing it is connected and you are still having issues isn't unusual, but often they alert you if you and a contact do/don't share the app. They may even allow you to compose messages to contacts that don't have the app, and will include or preclude the message with an offer to join/install the app. If sending a message outside of the app is impossible then the only error may be the "invalid contact" message.
You can tell us a bit more about the app and answer the questions I've asked by registering an account. This link should help you with that...
https://forums.androidcentral.com/as...-new-post.html
