You probably haven’t instantiated an instance of Boss_Run anywhere.
It appears that a StateMachineBehaviour will be instantiated by the Animator. I don’t remember when the Animator does its business relative to Start, so that could be the problem.
More generally, you probably don’t want to use FindObjectOfType<>() because it’s very slow.
[quote]
I want to access the speed variable in the script above in another script that is attached to the gameobject that is running
[/quote]I think you’re overcomplicating the problem. This should be pretty easy. It would be a good idea to go through Animator-related tutorials to see the best practices for changing a float like that.