In an enterprise environment, not all users has Administrator rights.
If you install ArcGIS Desktop in a PC and the user account intended to use the software do not have administrative rights, it wont be able to setup the access to Portal in your company.
check this workflow:
- Install portal in Server A therefore portal will be available at https://myServerA.mycompany.com/portal
- install ArcGIS desktop in the machine Client1, it is installed using the account PCAdmin (it has administrative rights)
- the user PCAdmin sets up license and connection to portal using ArcGIS Administrator
- user User1 (non-admin account) will use ArcGIS Desktop + Catalog using the content hosted in the portal, but neither Desktop, nor Catalog allows him to sign in on the portal, because they does not show even the portal login page, only the AGOL login page.