Instead of uploading the add-in Office 365, under Setting>Service & add-ins, try uploading the add-in in SharePoint Online Add Catalog site. To do this, please follow the steps below:

  1. Sign in to Office 365 portal with admin credentials.
  2. Go to Admin Center>SharePoint Online Admin Center.
  3. Once you are in SharePoint Admin center, go to App Catalog site, URL should be something like https://tenant.sharepoint.com/sites/AppCatalog 
    1. You will have to change "tenant" to your Office 365 tenant name
  4. Click Apps for Office on the left navigation panel to upload the add-in for Office apps.
  5. Click Upload and browser the path where the custom add-in is saved on your system.

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Once you have successfully uploaded the add-in, you should see the add-in under My Organization in Excel Online.