I made a tower of Hanoi. There's a platform (cube), disc and a pole. When I try to play it, the discs suddenly fall even though it's on gravity and freeze rotational on rigidbody. What will i do?

If the tower should stay at the same position, you could remove the rigidbody components and set the objects to "static".

A static object is a non-moving object.

You can change the static / dynamic setting with the little checkbox in the upper right corner of the inspector.