Character rig problem

Hello, so I'm trying to learn how to animate a character and im stuck on a very beginning.
For a training Im using a model with no animations at all - Strong Knight from the asset store (free model).
I was trying to adjust the model's Avatar, and as I enter the Muscles&Settings , a weird thing happens.
I can see the influence of moving a slider in the Muscle group preview in the scene window, but when I try to adjust it in the Pre-Muscle Settings, nothing changes in the Scene tab, but only in Game tab.
And when I switch back to Scene tab everything is grayed out.
I tried it on lots of models, even the Unity-chan - every time its the same.
In every tutorial I watched, everything works normal in the scene tab. What am I doing wrong?
Thank you in advance!