I think we should add 'you' to make the sentence clear. But my point is that after any prepositions ('for' here), you should use a noun or a noun phrase, not a clause. That is, it could be "I'm sorry for not being able to give you an answer today" or "I'm sorry for the fact that I couldn't give you an answer today" but as far as I know, these are not used in spoken language, so it should be "I'm sorry I couldn't give you an answer today".