This is a brand new feature and I suspect it isn't fully implemented.
On the Customize the Ribbon dialog it indicates that this is part of the BarCode group on the Insert menu... but there is no BarCode group on the Insert menu, even when you have a displaybarcode or mergebarcode field on the page or selected.

For the moment, I believe, you will need to insert displaybarcode fields by writing them manually using Ctrl+F9 to insert the field codes.

Note, when I inserted such a field it did not immediately display and it showed up only when I inserted another field. Pressing F9 with the field selected did not cause it to display. Pressing ALt+F9 twice toggling display of fields did get the bar code to display.

Which kind of bar code are you trying to write? It would be relatively easy to create a macro that would seek the text you want and create the bar code. That macro could be attached to a QAT button.