Unable to open the autostart file to enable RealVNC to auto boot
I assume you're running Rasobian Stretch.
No idea what you are talking about re directory / file. Standard lxde autostart file is '~/.config/lxsession/LXDE-pi/autostart'
But RealVNC server is normally started from a systemd service file.
The easiest way to activate it is from the Configuration Utility (Start Menu / Preferences) or
sudo raspi-config from the command line.
If you prefer to enable it manually you can use
sudo systemctl enable vncserver-x11-serviced