Symptoms
You receive the following error message when trying to shadow in the shadow taskbar or in the Presentation Server:
“Shadow failed. Error code 7051
(Error 7051 - The requested session is not configured to allow remote control. )"
Cause
This problem is also seen when using remote control in Terminal Services. This indicates an underlying problem at the operating system level.

There is a Microsoft Group Policy object (GPO) that disables remote control. This GPO also disables Citrix shadowing.
Resolution
See
Microsoft Knowledge Base article 278845.
- Open the Group Policy snap-in (Gpedit.msc).
- In the left pane, under the Computer Configuration branch, expand the Administrative Templates branch.
- Expand the Windows Components branch.
- Click the Terminal Services folder.
- In the right pane, double-click Sets rules for remote control of Terminal Services user sessions.
- On the Setting tab, click Enabled.
- In the Options box, click Full Control with users' permission and then click OK.
Note: The same setting also exists under User Configuration in the GPO. Also, the setting may appear in more than one GPO applied to the user(s) in question. - Run gpupdate on the Presentation Server so that the change takes effect.

- Test Remote Control again to ensure that it now works. Once shadowing works through Terminal Services, it should then work in Presentation Server.
Source