It's hard to say, but it looks like an issue with your frontend app code that is not finding the required asset or code snippet of the Quantity Breaks Now app (which is referenced in the code but no longer exists or wasn't properly installed). This typically happens after you have updated the theme to the latest version.
You could try removing the app completely (please use the app's de-installation instructions for that) and reinstalling it in your theme to see if that fixes it.
