RealVNC no longer sells or issues new license keys for version 5.x. Portal Removal:
, which allows a single license key to be applied across any supported platform (Windows, Linux, or Mac OS). This simplifies mixed-OS deployments by removing the need for platform-specific keys. OldVersion Key Features of VNC 5.x Licensing Universal License Keys vnc 5.0.5 license key
Are you trying to for an old installation, or RealVNC no longer sells or issues new license