Hmm. That is odd. Is this happening for the VM and YYC? Are you on Windows or Mac? Did you add TweenGMS to an existing project or an empty project?
Testing here on 2.3.3.437 I am not seeing the same error.
It is safe to comment out that line if it stops the error. I'll check things on my end and see if I can reproduce the issue.
Edit:
Looking things over some more, the GameMaker error doesn't seem to make sense within the given context. Something seems really wrong. I am suspecting something is wrong on GameMaker's end, as "health" is not associated with any attempted assignment. That would indeed be wrong!
I might encourage clearing the compiler cache or reinstalling the GameMaker runtime. Or, you could try a different runtime to see if the error goes away or changes. That might give a clue as to where the error is coming wrong.
Please keep me updated :)