I cant send imessages in my phone

Start by checking if iMessage is activated and if your phone number is active. Remember that your iPhone must be connected to your carrier before you can send iMessages to your number and receive messages on your number.

Go to: Settings → Messages → Send & Recieve → check if your number is active on both lists. If your number is not active, deactivate iMessage restart your iPhone and go back and activate again. Remember to be connected to Wi-Fi or Cellular Data.

You can find more useful information:

If you can’t turn on or sign in to iMessage or FaceTime - Apple Support