Why can’t I use my iMessage

When you communicate with others using iMessage, you can use more apps and features, like read receipts, undo send, message editing, audio messages, collaboration, inline replies, and more.

1. Go to Settings  > Messages, then turn on iMessage.
2. To select the phone numbers and email addresses you want to use with iMessage, go to Settings > Messages > Send & Receive, then choose from the available options below “You can receive iMessages to and reply from.”

If you have further issues, contact Apple support or feel free to reply here.

Additional information: Set up Messages on iPhone – Apple Support (UK).

Probably cause you are not signed in to iMessage or you need to update your phone