Hello zzzhhh,
Problem isn’t related to NoMachine software but to RedHat. Instruction of installation Xvfb (“sudo yum install xorg-x11-server-Xvfb”) is correct. You can find the answer for your issue here: http://serverfault.com/questions/344793/install-xvfb-via-yum-yum-repository-for-xvfb or by enabling rhel-7-desktop-optional-beta-rpms repository.
We will check specifically against the RH t2.micro instance and make sure the article is updated.