I've found this question across my JLPT N4 practice
1) 楽しかったですよ。参加（できなくて/できないので）、残念でしたね.
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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1But it doesn't look like a test on the difference between the two forms, but rather on tense agreement. – macraf Sep 25 '17 at 1:23
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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.
