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