What is the address to which I need to send my iPhone for repairs? A shipping label wasn’t included in the box. Thanks

A label should have been included. Contact Apple if one was not:

Refer to this document for ways to contact Apple --> Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support

Where you send it depends upon where you are: See Learn how to send your iPhone to Apple using Express Replacement Service - Apple Support