You can't make a screenshot if you're in the chat or typing something in a commandblock.
Linked issues
Comments 3
MC-29469 - Please use search function next time
This might be intended or unfixable, their first attempt to fix it ended up in even worse problems. I doubt this is intended, but for now use the Print key to save your screen into your clipboard and use programs like Paint to save them.
also view MC-32909 extra info of the bug!