I'm having the same problem, can't type any letters when trying to login using a PIN on windows 10 PC, my pin has some letters so i need to be able to type a letter, otherwise i wont be able to login using PIN AT ALL.
Are you able to login with a PIN that contain only numbers?
On the Login screen make you you selected the right Sign-in options (Password or PIN).
It could happen that Password is selected and a PIN is entered.
Windows 10 has a button on the bottom right of the screen, Ease of Access, which opens up a panel allowing you to bring up the on-screen keyboard. At this point you can use your mouse to type.