Overriding Tax Classification Code on Invocie Line in Project Billing Oracle r12

Hello:

I am trying to enable the Tax Classification Code so that I can override what it deafults to on Unapproved Invice Lines in project billing.

I have already set the profiel option eBTax: Allow Override of Tax Classification Code to YES but the field is still grayed out.

How can I change this field so that it can be edited, i.e. I can override the tax classification code on invocie line in project billing Oracle r12?

Thanks