In order to comply with Apple and Android requirements, you must provide a physical address on your digital stamp card.
If you don't have a physical address, you must specify an alternative address that you are able to receive mail from. Here are some ideas:
A P.O. Box where you can receive mail.
Rehearsal or studio space.
Sometimes the only answer is to purchase a P.O. Box, but hopefully this gives you some alternative ideas to work with so you can get started sending email campaigns. Whichever option you choose, make sure the address you use is valid, reliable, and that you're able to receive mail there.