Freddie here from the Shopify Support team, happy to help.
Which PayPal account are you trying to use to pay for the transaction? If you are trying to use your same PayPal account to purchase with, that the PayPal Express Checkout has been signed up with, then you may be seeing this error. Have you tried with a separate PayPal account to pay for the test order to be sure?
I'd also recommend you to reach out to PayPal support directly if you're having further issues, as they will be able to look into your PayPal account settings and be able to diagnose an issue for you and how to find a solution to ensure that everything is working as it should!
Let me know if this helps and if you have any further questions.
Shopify Support - firstname.lastname@example.org