Puerto Rico is not available under "Country or Region Availability" in appstoreconnect

What to do before you change your country or region

• Spend any balance remaining on your Apple ID. If you're not sure whether you have a balance, check your Apple Account balance.
• Cancel your subscriptions and wait until the end of the subscription period. 
• Wait for any memberships, pre-orders, movie rentals, or Season Passes to complete, and wait for any pending store credit refunds to process. You can cancel a pre-order.
• Make sure that you have a payment method for your new country or region. See what payment methods you can use.
• Redownload your apps, music, movies, TV shows, and books on any devices that you might use to play or read them in the future. Some types of content might not be available in your new country or region. Learn what types of content and services are available in your new country or region.

Puerto Rico is a US territory. It is part of the US.

isoCountryCode | Apple Developer Documentation

For Apple store, yes, Puerto Rico is part of the US, but for iOS development is a different "country".