Proteus requires elevated permissions to access license registry entries.
Are you using a student/trial license but trying to run "ISIS Professional"?
Navigate to: C:\ProgramData\Labcenter Electronics\Proteus 8 Professional (Note: ProgramData is a hidden folder). Locate the file named LICENCE.INI .
The error is most likely resolved by addressing permission issues or restarting the background licensing service. If the steps above do not resolve the issue, the license file may be corrupt or expired. In this event, contact the software support channel with your (usually found in the Help > About menu or a separate utility tool) to request a replacement key.