Hi Francis, this is a bug in version 1803 . . .
Cancel the PIN prompt and login to your PC
Go to Windows Defender Security Centre icon on the tray.
Under 'Account Protection' go to 'Set up Windows Hello for faster, more secure sign-in'
Click 'Set-up', it will prompt you to set up a pin - don't.
Click "Dismiss"
After that, there should be no more prompting at startup