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-With the latest version of MS Office, hover your mouse over the top right of the task pane. A ‘<' arrow will appear, click on 'Attach Debugger’. Attach a debugger from the task pane - Office Add-ins | Microsoft Docs
-With older versions of MS Office, you will need to get ‘Microsoft Edge DevTools Preview' from the Microsoft Store, in Windows. (not in the MS Word Store). Get Microsoft Edge DevTools Preview - Microsoft Store
Microsoft has outlined some of the common Add-In errors and troubleshooting techniques: https://docs.microsoft.com/en-us/office/dev/add-ins/testing/testing-and-troubleshooting
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