Invalid production license key or need to use FVLicense.exe
Run the FVLicense.exe program which has the title of Access Code Management in the Inventu Viewer Start Menu--Once it is loaded, select "Check Subscription" or "Check Maintenance" in order to connectd to the Inventu License server and validate your Access Code/License Key.
Note that if your server is "locked down" and has no outside HTTP access, you will need to use the clipboard in a Remote Desktop Session in order to validate the key.