The above error occurs typically when you try to install an add-in from the SharePoint Store using the O365 Global administrator account.
- GoTo Office 365 Admin Portal.
- Under Users, Select “Active Users” and Add a user
- Make sure that you choose “SharePoint Administrator” as the role for the new user
- Now try to login to the site using the new user and add the app from the SharePoint Store.
The above had worked for me when I tried to add the Napa developer tool to the Dev site.
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