1: Open iVend Management Console at store
Go To: Administration >> Retail Configuration >> Document Print Profile. Click on New Button.
#: Configure the Document Print Profile for the requested document, check below.
2: Open requested document and check this time print button should enable. If not, then restart management Console and check.
Above suggestion can be tried on iVend 6.6 any patch with and without SAP B1 9.3 also.