Hello ianyylai,

You have a beam inside a tube. You want the sides of the tube to hold the beam from falling over. Therefore, you have to add Frictional Contact between the beam and the sides of the tube. Without contact, there is nothing preventing the beam from moving in X or Z (assuming Y is vertical) or tipping over.

Also, if you don't have Standard Earth Gravity turned on, the beam is not pushing down on the surface where you have defined contact.