Is it possible that you’re using an older Shopify store URL, if the myshopify.com store handle was changed? This seems close to this recent issue which was resolved by using the updated myshopify.com url.
Is it possible that you’re using an older Shopify store URL, if the myshopify.com store handle was changed? This seems close to this recent issue which was resolved by using the updated myshopify.com url.