WACC - After tax and before tax

I came past a question in a mock today which wanted you to calculate the value of a firm using a one stage FCFF model. They gave you all the variables to calculate WACC including the tax rate. In the answers they have used the before tax WACC.

When should we use the after tax wacc and before tax wacc?

If this is PE V1, there is an errata on their website

thanks cmlsml