So folks.....Here is the issue with Live Messages failing to send on AT&T.
AT&T has what they call "Advanced Messaging" which increases the
send/receive limits from 1MB to 10MB but unfortunately because AT&T plays dirty they restrict factory unlocked US devices from being able to use this feature. You must use a locked AT&T carrier version for Live Messages to send properly regardless of whether you use the default messaging app or Textra. In the carrier version you have the option to enable "Advanced Messaging".