Whilst attempting to enable SSO within Virtual Center Appliance 5.5, you recieve the below error:

Error: VPXD must be stopped to perform this operation.

There are two options available to you, with option 1 being the quickest:

Option 1:
1) Click the summary tab
2) Click the stop button by the side of Server, wait for service to stop. Start the service when work is completed

Option 2:
1) SSH to the vCenter Server Appliance and type chkconfig (Press Enter)
2) The service name you are after is:

vmware-vpxd              on

3) Check that the vmware-vpxd service is on
4) Type chkconfig vmware-vpxd off
5) When done, turn the service on by typing vmware-vpxd on