sprint/T-mobile not linking to apple store

Upon speaking with Apple Support, I was informed to purchase the iPhone unlocked, not associated with any carrier. Once purchased, this can then be associated with the carrier of your choice. No connection discount will be applied for iPhone 14 only 14 Pro and higher.

No idea what you mean by that.

I am experiencing this same occurrence: preauth.account_not_found.error.body

I have also been experiencing this very same issue. Quite annoying when this process used to be seamless before the buyout.