Alternate Solution is to create a custom field (Body Fields or Column Fields) and make it mandatory. The custom field will require users to indicate if they have added a tax code in the transaction or not.
Below are the steps for this option:
1. Navigate to Customization > Lists, Records & Fields > Lists > New
2. Enter name of a list, ex. Tax Code Entered?
3. Enter Values = Yes and hit Add
4. Enter Values = No and hit Add
6. Navigate to Customization > Lists, Records & Fields > Transaction Column Fields > New
7. Enter label, ex. Have you selected Tax Code?
8. Type = List/Record
9. In the List/Record field > select the lists created above (ex. Tax Code Entered?)
10. In the Applies To tab > mark Expense Report or Expense check box
11. In the Validation & Defaulting tab > mark Mandatory check box.
12. Click Save