What do I do if I see “The recipient you are sending to is on your blocked list. To allow texts to this recipient, text 'allow ' to 9999. Msg 2134” in messages

i try to send messages to people with green bubbles & it never delivers and I get a message from 9230 saying “The recipient you are sending to is on your blocked list. To allow texts to this recipient, text 'allow <recipient>' to 9999. Msg 2134” and when I send messages to people with blue bubbles, my sent messages turn green! I don’t know what to do.