The {curly brackets/braces} occur in Word when it stops formatting the EndNote codes in your Word Document. This tells you the two programs have stopped speaking to each other.
You can normally fix this by clicking the "Update Citations and Bibliography" button in your EndNote toolbar in Word. Important- do not change anything within the {curly brackets/braces} before clicking Update Citation and Bibliography. As long as you haven't changed anything in the curly brackets, your reference will format to a normal output style such as JAMA or APA.
If you have changed anything inside the {curly brackets/braces}, you might need to remove your altered citation (by clicking Edit and Manage Citations) and put the correct one citation back into Word.
A second possible reason for the {curly brackets/braces} is because you have Track Changes on. You will need to not be viewing the Track Changes (Track Changes can be on, just not visible) to be able to format documents without the error messages noted above.
To work with Track Changes in a Word document turn off the viewing of this by doing the following:
To turn off the viewing of tracked changes, go to the "Review" tab in word, and to the right of "Track Changes," you will see a drop-down menu. In the menu, you will change this to an option called "Simple Markup" or "No Markup" (depending on your version of Word, you will only have one of these available on this menu). Once this is set to "Simple Markup" or "No Markup" try formatting the document again (clicking "Update Bibliography and Citations" button) and the error should not happen if it was due to this issue.