I’m a AT &T prepaid customer and why can’t I add cellular to my Apple Watch I got error not supported for my account type

Hi

Apple advises that pre-paid accounts (and some older accounts) are not currently supported for Apple Watch Series 3 (GPS + Cellular):

Apple Watch - Carriers - Apple

You may wish to submit a feature request to your carrier.

Because AT&T requires a post-paid account in order to do so.