Well, Penn Foster is mostly compatible with computers and I suppose their system has not fully expanded to make a more comfortable mobile device experience. Although most of the courses and accessories for said courses will work smoothly on PFHS mobile, certain tasks such as dragging specific items can be a difficulty. You could do the following:
1. Get on a laptop and do the task there.
2. OR contact technical support and see if they will be better able to assist you with this issue.