As described in this youtube video by “CodeWithChris”, the library pane in XCode 10/11 no longer remains open and ready for use. I’m not sure why Apple changed the behavior of the library pane, requiring you to re-open it each and every time?! To solve the issue, option click the icon and the library will stay open (although still in a separate pane)!
XCode 10 Library Pane
Nicholas Saraniti 