It tells me "You have a typo." However, I do not. It tells me I chose "Let s," but in fact I chose "Let's" which is the only choice available. Duolingo needs to correct their own error here.
Happened to me also.
I don't understand why the sing title is inserted in the middle of "let's sing together you and I" when clearly the verb cluster is first, a the song title comes after the "i".