A customer doesn't have a card
To enter a sale, you must begin by choosing a customer card. If you're entering a sale for a new customer who doesn't have a card yet, you can quickly create one using MYOB Accounting Plus's Easy-Add feature. Follow the steps in To enter information about a new customer (Easy-Add). Easy-Add enables you to create a card with only the customer's name and the standard default information that is provided for new customers. You can use the Card Information window to add contact information and make other changes later on if you like. (Adding cards)
If you frequently make sales to one-time customers and you don't wish to track information for each of these customers individually, consider creating a customer card called "Cash Customer" or "Walk-In Customer" and assign it the credit terms that apply to this group of customers. That way, you'll be able to enter sales for these customers more quickly and you'll be able to view sales information for them as a group.