Please unlock my iPhone XS Max

Use the unlocking portal.

https://www.att.com/deviceunlock/unlockstep1

You can try again later, like the message says.  However if you keep getting the message, it’s likely there is something in the input that keeps getting it rejected.  If you’re sure the IMEI and other data is correct, and the phone is eligible to unlock, then your only other option is to call AT&T and ask for advanced tech support.