The SumUp checkout require payment scope which is restricted from SumUp end by default, Please check SumUp Documentation.
Please contact SumUp support to enable payment scope for your account.
If you are still unable to accept payment after enabling payment scope.
Please raise a ticket at https://webkul.uvdesk.com/en/customer/create-ticket/
And on the ticket please share your QloApps Credentials and FTP details so that we can look into the issue.