Imei not recognised during unlock request?

Maybe it isn't locked to ATT.  Contact Apple and ask if it's locked and to what carrier.

Nope.  

It says AT&T as the carrier because the carrier settings is set to that specific carrier. Unlocked and unbranded iPhones will show that when they're setup properly.

If the IMEI isn't recognized, it's not a branded phone.

If it's an iPhone 6s, it is able to use ANY carrier.  GSM and CDMA.  

If it has now, or did have an ATT sim in it, it will say ATT.  Most likely still has a sim.

You can run the IMEI through swappa, it will tell you which carrier the IMEI is for, if it was ever locked to or sold buy a carrier.

The easiest way to know for sure is stick another sim in and power cycle the phone.