I can’t send iMessages
If everything else is working, I suggest you start with a resync to iMessages:
• Turn off iMessages in " Settings "• Wait a few moments• Turn it back on again• Then restart your phone
That's usually necessary if you make any changes regarding your phone -- such as changing to a different phone, changing to a different phone network or getting a new phone number.
But if anything else isn't working, give us the details -- it may help diagnose the problem.