I have been facing this issue with my phone for the last 2 days . I tried to change the password to swipe just to see how it looks. I didn't like it so I wanted to change it back to pin. since then I have never been able to change the screen lock to pin or to any other password. the only option left is to do a factory reset after which I will lose a lot of data which cannot be restored for example some apps which has a particular password which cannot be recovered even after reinstalling. I didn't expect this kind of a Glitch from Note 8. it's a damn expensive phone and it's really inconvenient.