Hello! This problem can be caused by different things, so let's review that everything is correct with your phone by following these steps:
Go to Settings > General > Date & Time and make sure that you have your time zone set correctly
Try to restart the iMessage feature by first going to Settings > Messages and turning off iMessage, restart your phone and re-activate iMessage.
If the problem persists, check that your phone number is correct. While in your Messages app, press settings when the prompt appears and follow the instructions or fix the errors that are shown.
Contact your carrier, to check that your call and sms features are correct and that your sim card is active.
I hope that this helped you solve your problem, don't hesitate to ask if you would have any further questions!