Please help!!!  Very simple VI job has been in use since 2015, maybe earlier.   But within the last year, we have had at least 3 incidents, and want to make sure that if user forgets to populate this column of the AR cash receipts excel template, the record errors out.  The invoice type within the batch can be IN or PP, but should never be blank.

What is the simplest way to approach this?  Can the Conditional expression in the import field properties tab be used to validate that source data is IN or PP and never blank?  I tried to set up a conditional expression and cannot get past the syntax errors --  Is there some kind of manual for VI beyond the overview that explains syntax for conditional expressions?  Thanks very much!