Can anyone explain how to implement the loop in CMMN model?
So I have a Stage A with one human task A. When the task is finished, the case moves to Stage B.
In stage B we have another human task B. When Task B is finished, I want to move the case to Stage A and repeat it with human task A.
Loops in CMMN can be defined with repetition rules, see https://flowable.org/docs/userguide-cmmn/index.html#_item_control_repetition_rule
But reading your use case, it looks like you simply need to have a sentry on Stage A that listens to the completion of B. Note you can have two sentries: one for this, and an empty one (could be you need an expression that simply evaluates to true) to allow for Stage A to be created the first time.