Unable to apply prepayment

ERRO MESSAGE:The tax couldn't be calculated. you can't apply the prepayment invoice as the withholding tax classifcation code on the prepayment invoice and the standard invoice dont match

I have two invoices that is standard invoice and prepayment invoice, where i have applied tax on standard but not on prepayment, now i am not able to apply that prepayment invoice on standard invoice for the obvious reasons because standard invoice has WHT and prepayment doesn't. Both invoices is validated.

So i tried to add tax going in "edit taxes" and added a tax in the same. tax is added but then also it is not applying, but amount is changed.