That really should run fine, I don't see how that's giving a syntax error, unless it doesn't like the indentation. In any case I defer to Dan's solution which is running the same thing but in one line:
!NADR_ID! if !NADR_ID! in !ABOVE! else None
I realise that should be "NADR_GID" not "NADR_ID", where are you putting that line of code? It should be in the box below the code block, and the code block should be empty! If that fails, I'd be happy to take another look at the error message it gives.
Also I realised in your previous screenshot it's running as Python9.3, try the above line or my original solution with "Python" as the expression type just in case!