Number not available

I recently upgraded to iPhone 10 but just before a factory reset of my old iPhone 7 I noticed when trying to send message from the 10 that I get a (number not available. Which number would you like to send the message with ?) Message, primary is the only option but get an red error next to the message. I’m using my 7 to send this message & don’t want to factory reset unless I can use my 10. I’ve went to all the settings on the 10 & can’t find anything about primary number. Please help me understand what this means and how to fix it, I have 8 days now to decide if I want to keep the 10