Purchasing iPhone with Boost mobile plan. "The address you have entered is not valid."

Hi everyone, I’m attempting to purchase an iPhone with the Boost mobile plan, but I’m unable to proceed past the personal details page. I’ve tried everything, including visiting the Apple store and collaborating with Apple specialists, but I consistently receive the same error message: «The address you’ve entered is invalid.» The store representative informed me that he’ll report this issue if it’s resolved within a few days, but it’s been over a week now, and the problem persists. Could someone please assist me in resolving this issue or provide guidance on how to report it to Apple for further investigation and resolution?

Thanks.