Order Customer must be an Active Customer - System Option

>Utilities>System Options>Order Entry>Order Entry

Category-Configuration

Type- Can be Global, Assigned or User

When this option set to yes, the user can only select from customers that are classified as “active” within the customer file.  Require Order Customer to be in Customer File must be set to Yes before using this feature. With this option set to yes, if listed, inactive records will show as gray in the customer list browse.

This system option also applies to Customers in Bid Pro. 

The active customer box has not always been a part of the customer file, to make all customers in the customer file active go to Order Entry>Utilities>Fix>Activate Customers.