Even if they enter correct PIN twice.There is no way to go forward, and backward.

This is part of the account setup, and the computer should stay connected with the internet before everything is finalized. While PIN works even when the computer is offline, the account setup definitely needs an Internet connection. At least for a fresh installation.

Windows installation stuck on Set up a Pin

The solution is rather easy. Since your installation is complete, and all that is left is account setup, even if your computer reboots, or shut down, there will not be any problem.  However, the first thing you need to do is turn off the internet or disable all connectivity from the computer. If there is a WiFi switch or ethernet cable which you are using to connect to the internet. Disable and remove it. In case you don’t have any of options, its best to turn off the internet completely.

Next force your computer to shut down, and then reboot your computer. The setup process will pick up exactly from where it left off. The only change is that there is no internet connectivity. This time the screen will not be stuck on the PIN setup screen, and can also skip it. So wait till you are logged in for the first time. You can then choose to Setup Pin later. Later, you can turn on the internet connection, or connect your computer with an internet connection. These issues are really silly in nature, and Microsoft needs to fix or at least give an option to skip where the internet is not really a necessity.