・・・。
Sorry if you didn't get the punchline...
Literal
Punchline [gen-の] meaning [subj-が] understood-not [conditional-たら] sorry...
オチ 'punchline / twist ending' is the Kansai-originated comedy term now standard nationwide. 意味がわからない 'can't understand the meaning' is the standard incomprehension phrase. すいません is the casual contraction of すみません. The trailing ・・・ creates the apologetic, trailing-off tone of someone unsure if their joke landed.