Cannot edit the sales tax for a purchase order

Hi gurus.

When i created a purchase order, normally the sales tax is calculated automatically, default by 17%, but for some special case, we have to change the percentage/actual amount manually.

But when i open the purchase order line and click “Sales Tax” on the action bar, seems everything is gray out on the “Sales tax transactions” form, i cannot change any value there.

do i have to enable some features to make this work ?

many thanks.

Can anybody share any idea ?

I am not sure about the PO part, but on the vendor invoice, on the financial tab, if you click on Sales tax, there is an additional tab for adjustment. You can override the calculated sales tax amount there. The feature should be available on PO level also, but i don’t see adjustment tab there. There might be some setup or config key.