OneSignal prints helpful error messages in your browser's Developer Tools Console when viewing your site with the OneSignal code active. Follow these steps to open the Chrome Developer Tools Console and check for errors on the site:

Open the browser's Developer Tools Console with is F12, or right-click the browser and press "Inspect", or go to "View > Developer > JavaScript Console".
Make sure you are on your website with the OneSignal code enabled.
Subscribe to your website if you have not done so already and check for the following errors:

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