Hi @yemin64270, thanks a lot for posting at our forum 
When creating a post on the forum, make sure to always share the project link, if possible! In your case, since yours was public, I could find it on your profile:
As to your question
It seems that you’ve set a fixed support boundary condition to that surface, that’s why it doesn’t move
The fixed support will restrict movement in x, y and z directions.
Regarding this part
I guess you could apply a formula and select the value in kg for examples then multiply it by the gravity magnitude (\mathrm{g \approx 9.81 \; m/s^2})
If you think that should be a native feature within the platform, please submit your suggestion at the SimScale RoadMap 
Best,
Igor