There seems to be some confusion regarding whether the 2021 Buick Envision can be flat-towed. I have seen comments saying that Buick discontinued the flat tow capability in the Envision after 2020. Apparently, that model has an upgraded transmission for 2021. It appears that the Buick Encore remains flat-towable in 2021, however.
The 2020 and previous Envisions could be flat-towed. Neither of the two major baseplate manufacturers (Roadmaster and BlueOx) offer base plates for the 2021 Envision, however. Both offer them for the 2020.
You might want to inquire of Buick’s engineering folks whether the 2021s can be flat-towed. Asking a car salesman for that information likely won’t get you a definitive answer.
TJ