When I send a text message I immediately get a message not delivered messages for message blocking active? How do I make this stop

The message is being blocked from the recipient's side. They need to figure out if they have you blocked on their phone or through their carrier.

When you receive the "message blocking is active"-error, it means that the number you are trying to text has been blocked in some way. It could be that they are in your block list or you have some sort of parental control set up.