Withholding tax setting

Der experts !

I enabled withholding tax management in General Settings and in Item master data. Also I defined a withholding tax.

When trying to enter a tax, on a AP invoice, the fields 'WTax Liable' displays 'No' and it is unchangeable ('Yes' is no longer avalilable here). How can I do to change this field to 'Yes' such that the amount of tax is displayed on the document footer (field 'WTax Amount')?

Please help me solve this problem.