If you go to the cog wheel top right> under settings is custom form styles> click into the template you use>content>click into the second box on the right> scroll down to and on the bottom left there should be a box to tick that says item code> tick that and then when you go back into an invoice-print/preview the codes should show up for you.
If you go to the cog wheel top right> under settings is custom form styles> click into the template you use>content>click into the second box on the right> scroll down to and on the bottom left there should be a box to tick that says item code> tick that and then when you go back into an invoice-print/preview the codes should show up for you.