Text Message "Not sent. Tap to try Again"

I too have had this same issue. Doesn't matter how many times I try to resend. I have also tried using a different text app, switched to 4g only, restarted my phone, cleared the cache and still will have the same issue. I will do a test send to myself or another person and usually but not always have no issue. So I think maybe it's that person but I will immediately send another random msg to them with no issue. 

So, figure that one out T-Mobile because it's highly irritating especially when the text is important or in the middle of an argument.  And please don't tell me to do something I already mentioned I have done probably many times.