getting error - The link you have used to enter the PayPal system is invalid. Please review the link

Hi,
I am new to setting up membership sites. I am currently trying to test the subscription using the paypal express sandbox site.

When I click the checkout button it says the following when it reaches the paypal site

"The link you have used to enter the PayPal system is invalid. Please review the link and try again.
"
Could you please guide where might be the issue

thanks