After more than 15 years covering everything Apple, it’s with a heavy heart we announce that we will no longer be publishing new content on iMore and the iMore forums will be closing as of November 1st, 2024.
Verify your iMessage service is enabled through Settings > Messages. If it is trying turning it off and back on to allow it to reactive. If that doesn't resolve it, verify the person you're trying to message has iMessage enabled as well.
Are you able to send iMessages to other iPhone users?