Why can't I save a change to a subscription because of the address? (Essential Subscriptions app)

When you change a subscription's schedule, Shopify checks the delivery address and delivery method again. If something in the saved address doesn't pass Shopify's checks, the change is blocked, even if the address looks right.

1. Open the subscription and click the edit icon on the Customer card.
2. Enter the full delivery address again, including the country and phone number, and save it.
3. Then make your schedule change.

If you see "Shipping Address phone enter a phone number" when you create a subscription yourself, your store requires a phone number at checkout (Shopify → Settings → Checkout). Set the shipping address phone number to optional while you create the subscription, then change it back. See How to Create a Subscription for a Customer Manually.