I've found this question across my JLPT N4 practice
The answer is できなくて What are the differences between ので and て in usage for reasonings?
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The の simply adds an explanatory tone to the sentence.
今忙しいので、どうぞお先に行ってください。 (As I am) busy right now, please go on ahead.
But I think you're veering away from the original grammatical matter being tested in your practice sentence. macraf is giving you a good hint.