I have no idea where that help page took those screen shots from... but it is definitely not from github.com or github enterprise. I see no way to assign pull requests... i don't even see the checkbox they mention on the PR list!
You need to have someone to assign the PR to. Either you need a second contributor on the repo, or you need to be in an organisation with more than one member.
That's really odd.. I see it on both the public github and the not completely up-to-date GH-enterprise setup I have access to...