You can get this error while submitting the "Update Card" or "Renew" form in the experience site. This is only for the form that opens up from the Membership updates component.
If you check the Payments2Us error logs you might also find a error logged with the description text containing phrase like "You don't have select access to object Campaign"
This error comes because some experience cloud licenses doesn't have access to all standard salesforce objects like Campaign in this case. You can check the standard object access for different Salesforce license in this Salesforce documentation.
If your experience user license type doesn't have access to campaign object then please remove "Campaign" fields like "Default Campaign" from the payment form.