The symptom
When no rate applies to the address and items in a cart, the customer may be blocked before payment. Start with a real test using the product, quantity, and postal code the customer reported. Do not change the entire configuration before reproducing the issue.
1. Zone and address
Under Settings → Shipping and delivery, open the relevant shipping profile. Check that the destination is in a zone and that the zone has an applicable rate. A store can sell to a country while lacking a valid shipping option for a particular address.
2. Profile, location, and product
Check which profile contains each item and which fulfillment location can handle the order. A product in a custom profile may not use the rates in the general profile. Test with one item first, then with the original cart combination.
3. Conditions and external services
If a rate depends on weight or order value, test the exact threshold. If it comes from an app or carrier, check what happens when that service does not respond and review the backup options available to your configuration.
4. Retest before telling customers it is fixed
Test several addresses that should work and one that should not. Record the original case, the change, and the result. If the issue remains, take that evidence to Shopify or the shipping app’s support. The goal is to fix the cause, not hide it behind a generic rate.