Just replaced alternator on my GMC envoy and it’s saying battery not charging

Testing is needed. You'll need a good voltmeter / ammeter to see what the voltage and amperage output of the alternator is. "New" alternators have been known to be bad out of the box at times. The wiring connector at the alternator may be damaged or melted and should be inspected. Check the belt for proper routing and tension. Something will turn up that isn't right if there is truly a problem.