You should not require a separate license key for your build server, so long as the installation is being performed by an already-licensed user.

There are a few common solutions for this issue:

• If you typed in the license key by hand, try coping it from a plain text document instead.  It's easy to mistype a single digit.
• If you were copying the license key from Microsoft Outlook or Microsoft Word, I've seen situations where the key is treated as invalid.  Try pasting first into a plain text document and then copying from the text document, or entering the key by hand.
• If the above two approaches don't help, you can complete the installation and then run the installer again.  You'll find an option that will let you put in the license key, thus upgrading the trial installation to a licensed installation.
• If this also doesn't help, uninstall the toolset (assuming you completed the installation), reboot the computer, and install it again.  This clears out what we believe is a "cached," invalid value that is being used in place of often-valid input.

If none of this resolves your issue, then my best advice is that you submit a support request, detailing what steps you've taken so far.  A Developer Support Engineer will look into this further and provide assistance.