This may be too late for OP, but I just ran into that problem also. I could text a friend on the iPhone with no problem, but could not do this from Messages on my mini. The target would turn red, and keyboard would not send the message. I am guessing my friend has an old android.
Recreated the contact card with no luck
Then on iPhone messages app , toggled the send & receive targets as some suggested with no change
Finally went to RCS Messaging and deselected the End-to-end encryption and that did work