Drastic changes in the system date or time can trigger anti-fraud mechanisms, causing the license to appear invalid.
If you are working on a PCB design or circuit simulation and see the error you aren't alone. This is a common hurdle for users of Proteus Design Suite (the software package that includes ISIS). Drastic changes in the system date or time
Troubleshooting the "Cannot Find a Valid License Key for ISIS Professional" Error Drastic changes in the system date or time