This error message appears when a customer tries to access Gift Reggie using "/apps/giftregistry," but they have, or previously had, another Gift Registry app installed in their store. In this case, Shopify appends "-1" to the Gift Reggie URL, resulting in "/apps/giftregistry-1." To resolve this issue, the customer should navigate to Shopify Admin > Settings > Apps and sales channels, click on Gift Reggie, and customize the app proxy URL by removing the "-1."