Itop VPN's licence server will reject it. Nothing else happens. The danger is not from entering a key—it is from downloading key generators or cracked installers.
At these prices, the security risk and legal headache of a cracked Itop VPN licence key are simply not worth it.
Do yourself a favor: uninstall any cracked VPN client you have installed. Run a full antivirus scan (Malwarebytes or Windows Defender). Then choose a legitimate, privacy-respecting VPN service.
Using a cracked licence key violates Itop VPN's Terms of Service. While the company is unlikely to sue an individual user, they can: