The simplest way is restart your device. That really works for me. It seems that just an unexpected bug or crash.

Hey buddy. Tea Time App is extremely fantastic app demo for new app-building feature of Swift Playground. The code snippets they give are enough to show the basic new features of Swift Playground 5 but not enough to build the entire app. However, I noticed that the entire structure of this app demo and resources are shown in this video. Maybe you can try to DIY.

As an undergraduate student major in Computer Science, I must tell you that what you learn is not enough for your goal. For know more about development, you should learn more 'basic' language such as C. You can read a very classical book called 'The C Programming Language'. Also, you can't think that just programming language is needed for software development. Algorithm, Data Structure, Computer Networking and Operation System are all very significant in developing. You must learn these basic but helpful knowledge if you want further progress. For iOS software developing, you can read Apple's Docs and visit Apple developer website to search for WWDC videos. There are also many videos in YouTube. Computer Science is not an easy subject. I encountered many difficulties when I studying. I hope that you can keep your passion and make more progress. Good luck for you.😄