Hi Henintsoa,
I understand that you want to know why you're getting the error message 'Cycle detected ub rigidity graph'.
This is because SimMechanics does not allow creation of closed-loop system unless there are Joints present in the assembly. (such as a four-bar linkage).
If there are no Joint blocks in the assembly, you need not close the loop because it will not affect the behavious of the rigid body. For example, if you are creating a triangle using three bars 'A', 'B', and 'C', the connections 'A-B' and 'B-C' will suffice. Creating the connection 'C-A' as well will cause this error to be thrown.
Hope this helps!
