Requiring an invoice signature before issuing a payment order can be configured in the menu Company – Settings – tab Modules – tab Purchase. Here, check the option Require signature before issuing a payment order.
Signing an invoice:
open the Received Invoices form from the Purchase module,
above the invoice list, click the arrow next to the Services button and select the Sign for Payment option,
in the record list, the Payment Order Signature column will be checked for the given invoice.