Formula Doesn't Work in Custom Field

Here is a formula that DOES work in a saved transaction search, but does NOT work in a Transaction Body custom field:

{SalesRep.FirstName} || ' ' || {SalesRep.LastName}

Here are the key attributes of the Custom Field:

 Label:        Sales Rep FullName  Type:         Free-form text  Store Value: NO  Show in List: NO  Record is Parent: NO     Applies to:    Sale, Opportunity  Display Type:  Normal  Default Value: {SalesRep.FirstName} || ' ' || {SalesRep.LastName}  Formula:          YES 

The Custom Field returns value of "ERROR: Field Not Found" in both a Search and on a Invoice form.

My objective is to display the Sales Reps name as "FirstName LastName" on a PDF output of the Invoice form.