Reply To: I want to model this structure as Assembling and Dismantling Mechanism.

[peteroznewman]

Item 3, the columns meshed with beam elements are the cause of the error.
When you release all rotations at both ends, that makes the column free to spin on its axis, causing a pivot error.
You must constrain the free rotation of the column about its axis on one end. The other end can have all three rotations free.