Each product key can only be used once and on a single computer. If you see this message, then it has already been used and has been tied to a specific computer.
Are you actually trying to move the license (as opposed to activating a new license)? Check out the FAQ on moving a license.
Often, we find that if somebody is attempting to activate a new license and they get this message, it's because the product key was sent to several people (or passed along from person to person) and someone along that chain installed the license on their computer.
So try backtracking to see who has possessed the product key and enquire if any of your coworkers already used it. Once it's been located, you can move the license to your computer.
There are occasions where an error occurs during the process between activating the license and actually installing it on the computer. A different error would have been shown, but many people just click 'Ok' and then try activating the license again. This time, they'll get the "previously activated" error.
If you think this is the case, please contact email@example.com for further trouble-shooting.
Any questions? Please contact firstname.lastname@example.org