I’m not following your train of thought. The AT-6 does not have an airworthiness certificate because the process is expensive and a buyer (or “launch customer”) is yet to be found to absorb those costs since the USAF—so far—will not.
Armed aircraft can only be certified by a military authority. The FAA cannot certify the AT-6 like they did the T-6.
The USAF has not purchased AT-6s in any numbers, so they have not certificated it, either.
The fact that it is uncertificated in the US is a factor in evaluations, but is not driven by the lack of ability to be approved, which is how I read your comments. Please correct me if this impression is incorrect.