If you get the 'message blocking is active on iPhone' message when you try to text someone, there are two potential meanings. One, you may have unknowingly added the person you are trying to text to in the block list of the iPhone. Two, the person you are trying to text may have blocked your number on iMessage
This means you do not have text (SMS) sending Plans with your Carrier. You may only have Data Plan or Voice Plan, no SMS Plan.
Carrier has blocked and disabled your SIM to send text message (SMS), hence can't send or receive any text (SMS) messages. However you can continue to send iMessages.