I've had this problem for years. My iPhone won't sync up to my MacBook, although I've got iCloud and plenty of space. But the main thing that bothered me was that I couldn't create a new note! And I JUST got a solution. Untick the box with "resume last quick note". Cmd N doesn't work for me, and it is still greyed out in the menu-bar.. BUT.. If you move your cursor in the right corner (that is where I have a shortcut to quick notes at least), it pops up an option for a new note!