If an order or invoice will be paid with a credit card, you should have an account at Authorize.net to complete the process on SelBuk. Please to activate it on SelBuk do the following 

Main Menu > Database

Touch Sales - Invoices 

Touch Unpaid Invoices-Orders

Select an invoice 

On Payment Method touch Cash > Select Credit Card Now > Done

Then on Account type > touch Authorize.net label to type in the Login and Tran_key given by them > Done