You cannot remove the docking station when the computer is in the power saving mode. To dismount the docking station, you have to resume the computer to the normal mode, and then detach the docking station.
The online manual available in your computer describes the following:
"If the computer is in the power saving mode, you cannot remove the Docking Station. First switch to normal mode, then follow the steps below."
* The above details are abstracted from [Basic Operation] -> [Using the Docking Station] ->[Connecting/removing the Docking Station] -> [To remove the Docking Station]