ANSWER

This PayPal error is most often caused by the forwarding type used for your domain to your shop website.

This is usually caused by using a "Frame" to forward your domain to your Shopify store. You can fix this by switching to using CNAME Aliasing rather than a Frame.

Contact your Domain host providers and ask them to change the forwarding type from Frame to CNAME. The CNAME should be the name of your shop, and the myshopify.com address. Example: vogue.myshopify.com/