Unable to add sub item under page

hello everyone.
I have a website https://www.webzcreator.com/ and i am facing a big problem. i can not add more sub items under pages.
Initially i can add these items. you can see under service section. but now if i try to add more items under course section, then just blank box create, and not sub items added.
you can see in this pic. when i try to add sub items under course, then this type of blank box created.
kindly help me to solve this problem and help me how to add sub items under my pages.