In case you got this error when registering Zend Guard, you can find troubleshooting tips in the following KB article:
What to Check for an Invalid License When Registering Zend Guard
For Windows users,
The most frequent resolution is to run Zend Guard with administration privileges and register from local license file.
Also, the license must be a simple text file (copy from your account into notepad, then save as "zend_guard.zl").
In some rare cases, users reported that they needed to uninstall Zend Guard, restart Windows and install a fresh copy of Zend Guard to make the license registration work. This is probably due to user permission changes or system updates.
If you have went through the KB article and still didn't manage to register Zend Guard (with a valid license),
please create a support case and specify exactly what you already tried and results / errors.