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 
    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 browse to the path where the custom add-in (file found at the bottom of this article) is saved on your system.



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