I'm Ann, from the team here at Shopify :)
If you're having trouble accessing/using the Barcode Printer app, first off, ensure that you are using the latest version of your browser and that you aren't using any extensions/plugins as these can negatively impact the app.
Secondly, make sure that you are not using Firefox or Chrome as these browsers are no longer supported by Dymo. We recommend Safari for Mac and Internet Explorer for PC or another alternative browser. Additionally, you can also use the Dymo software instead of the Barcode Printer app! You can see more information about that here.
I hope this helps! If you tried all of the above and still have issues, there might be something else happening here. In order to take a closer look, reach out to us directly here :)
Have a great one,
In case you are sick and tied with the Barcode Printer app -
There is another option :
ReportJet ( Excel Templated Reporting App)
it has a report with Barcodes : 'Barcode Price Labels'
barcode labels are produced in Excel template and can be output to HTML (screen), PDF and Excel
and printed to (almost any) system supported printer.