Expense Reports _ Tax Inclusive ??

I am trying to "Create Configuration Owner Tax Options", for the event class, AP Invoices - Expense Reports. The "Allow Override and Entry of Inclusive Tax Lines" is blanked out and I cannot enable this option? Why is that?

We process a material amount of employee expenses that do contain VAT (reimbursable T&E expenses that are invoiced to the company, which the employee pays).

The current tax setup for "Expense Reports" defaults to a tax inclusive calculation. We would prefer these to be tax Exclusive because of the large volume of transactions that we process. This is my preferred solution.  How do I enable this?