Make sure the phone has an Internet connection via WiFi, Cellular Data, etc. 
Make sure that the AppleID and email are correct under Settings/Messages Send&Receive.

https://support.apple.com/guide/iphone/connect-to-the-internet-iphd1cf4268/16.0/ios/16.0
https://thinglabs.io/text-messages-not-showing-up-on-iphone
https://www.rd.com/article/why-are-my-texts-green/