Yathender is correct about your problem but the screenshot he used is not what you will have from the exercise. The exercise must be done from App Studio (as per the instructions) and his screenshot is from Dev Studio. If you follow this screenshot you create the Service case as a top level case (spin off case) not a child of the Assistance Request case as the exercise requires. Simply selecting the Create Child case(s) radio button here will not allow you to choose the Service case to be created as a child, there is additional configuration needed to make it available as a child case. When done in App studio you have no option for top level vs child case, it always does child case and will also take care of the additional configuration I mentioned earlier.