I cannot disable lock screen. When I try to go to settings -->security-->screen lock the none option is not there it has just been disappeared. I have a nexus 5 with android 5.1 build number lrx2cc
I think this is a duplicate of How can I disable the screen lock password in lollipop?
Basically, you should be able to go to: Settings -> Security -> Deactivate Administrators. Then deactivate anything listed.
Next, go to: Settings -> Security -> Clear credentials.
It should work now.