Can't Enable "Allow Withholding Tax" at Supplier Site Level

Hi, I've enabled Withholding Tax in Payables Options and at the Supplier level. I've also created the Withholding Tax Code and Special Calendar.

But at the Supplier Site level, the Allow Withholding Tax checkbox is greyed out. The system tip says it's only enabled when Withholding Tax is turned on in both Payables Options and Supplier — which I’ve already done.

Any idea why it’s still disabled?

Thanks!