When trying to use Sparrow inside Google Slides, the add-on will continue to load, until eventually a "Content can't be loaded" error message is shown.
Unfortunately, this is due to an issue with Google's add-on platform, and impacts all Google Slides add-ons that leverage the same Workspaces interface as Sparrow.
Some users have reported the add-on will work if you:
Open the add-on in a slide presentation that has less than 4 slides
Refrain from adding new slides to the presentation
If you have already seen the error message, you might have to clear your cache before the above workarounds will work.
If you want to see this issue fixed quickly, please lend your support on Google's Bug Tracker by starring the issue on the link below.