This button does not appear to be any where while editing a discussion?
I can give you a moderator or admin id if that helps.
what do you mean by lock
close (prevent posting or commenting) or announce ( sticky or keep discussion at top of topic list).
both are in the dashboard permissions for the roles - just check the appropriate boxes for admin role (or moderator) in vanilla 2.1.11
I may not provide the completed solution you might desire, but I do try to provide honest suggestions to help you solve your issue.
Thanks. I just wanted my first comment to stand by itself. So no one else would add to it. Probably a message would work better.