When a delegated partner administrator tries to access the SharePoint Online Admin Center site collection of one of the partner's clients in Microsoft SharePoint Online, he or she receives the following error message:
External sharing is disabled for the site.
To resolve this issue, use the SharePoint Online management shell to delete the external user account from the client's organization. To do this, run the following cmdlet:
For more information about this cmdlet, go to the following Microsoft websites:
This issue occurs when the account that the delegated partner administrator uses for sign-in was originally invited to the SharePoint Online site of the partner's client as an external user. External user permissions override a delegated partner's permissions. This prevents external users from accessing the SharePoint Online Admin Center site collection.
For more information, go to Partners: Build your business and administer your Office 365 partner account.
Still need help? Go to Microsoft Community.