Symptoms

You get an error Error: License key is used by another farm when trying to activate the farm:

Cause

The subscription license key can be used for licensing only one farm at any one time.

Resolution

Please deactivate the old farm prior to activating the new one: http://kb.parallels.com/en/124057

If you would like to keep several farms activated with your license, you can issue a sublicense key and use it for this purpose: http://kb.parallels.com/en/124774