I notice "yes, I'd love to!" is accepted, but why is "yes, I'd like to!" not accepted?
The "det" would be for "that" or "it". The expression in Norwegian is used to let someone know that you would like that/something which he/she suggested to be done/happen. Remember, you cannot always do a word-by-word translation. "Det" is a bit of a fill-in word here, but it would not be standard to omit it in this sentence.