I installed REALVNC VNC(R) Server 6.2.0 (r29523) ARMv6 (Aug 3 2017 18:32:53) in my Pi 3b board. The VNC server can not auto start when Pi reboot. I have to type the command " vncserver" to start VNC server, after that I can connect Pi from my PC through the VNC viewer.
In menu Preferences > Raspberry Pi Configuration > Interfaces, SSH and VNC are set to Enabled.
Run the command
sudo raspi-config, and navigate to Advanced Options, there is no option of VNC.