Some payment gateways sometimes require Thank you page link to insert in Gateway settings like PayPal.
Every thank you receipt URL has 2 unique parameters: order key and order id. The right combination of those 2 is required to open a page.
With order id and key in URL, every URL is unique.
When a payment request call is made, success URL is sent to the gateway as well. So that, after successful payment, the gateway should redirect the user to the success URL.
So, you need to insert any Thank You page URL in there. And test a real order/ transaction in order to see Thank You page is showing or not.
If correct Thank you page is not showing then raise a support ticket here.