What does it mean when you cant yell in your dream?

The body releases chemicals which dull or nullify your response to some outer stimuli while you sleep, and they 'disconnect the line' between your body and your brain. This is what stops most people running around or screaming while they dream, and what prevents you reacting to yourself. A person who is screaming in their sleep is in a deep enough sleep that this setting of sorts is active enough for them not to notice it happening.