There are many challenges that one might encounter as an engineer, but what is challenging for one person may not be challenging for someone else. For me one of the most challenging things is knowing what to do when you get stuck. There isn't a manual you can flip to and get the right answer from. In fact, sometimes the right answer is there isn't a right answer, or the right answer isn't possible right now. Sometimes we are limited by what we and our peers know and are able to accomplish. And to learn how to think around that and figure out other possibilities is extremely challenging. But it's also what makes engineering so interesting and fun. And like anything in life the more you work at something the better you get at it. So something that was challenging in the past may not be so challenging now because of practice.