I have a problem with Apple Private Relay.
If you used an Apple Private Relay on Kickstarter (or any other crowdfunding platform), it's not possible to use it on Gamefound. You can change the email on your Kickstarter account to enable the creator to import your cart.
Each case of such a problem has to be handled individually with the help of the creator. You can contact the creator using a contact form dedicated to a project.