If the error looks like this:
Then it means that either the installer was unable to copy the file to you system, or that the file/directory has been subsequently deleted.
There will be a second message, indicating that the Add-In has been disabled.
To fix the error you will need to re-install the software.
If that doesn't work, it may be that there are permissions issues impacting the installer and you should raise a support ticket HERE
(Making sure to copy the invalid Key message into the Ticket)
Our technical support team will be able to help further.