I have the Moto Supra g7 and here is how I finally figured out how to delete the text message from the unknown sender:
1. Open the message from the unknown sender
2. In the upper right hand corner there should be the dots which you should press to see what options you have.
3. Select "Block Unknown Sender".
4. The text message should then disappear from you list of text messages.
It worked for me....hopefully it will work for you.
I have a Moto Supra g7 and had the same problem....text message from an unknown sender and i couldn't delete it.
I finally figured out how to do it. Here's what I did;
1. Open the unknown message
2. At the top right click on the dot(s) to give you a dropdown of options
3. Select "Block the Unknown Sender"
4. The message should then disappear.
I hope this helps!