If you've clicked Sign in with a Microsoft account instead link and converting your local account to Microsoft account, then you would be asked to set up PIN number and it is mandatory.
The reason behind this is as follows. A PIN number is usually easy to sign in because your Microsoft account email ID password may be complex or longer and you probably don't want to enter it again and again to sign in your system.
Hence, you can proceed with setting up a four-digit PIN, which is easy to remember and
more secure than a password. You can
change or remove this PIN later, if you like.
