Hi,
I can edit that field in BC Saas ES version, NAV NAV2017 ES, NAV2016 ES and BC 14 CU17 at least...
In Sales Invoices, Purchase Invoices, CR, Service Invoices...even in the Posted Documents

The order of the fields is different in my side, which version are you using and CU?
Did you customize anything that could affect here? Or are you using the Extension for SII from a third party?
Thanks,