Sorry for my bad English.
I was listening the music Seaside Rendezvous from the Queen and in this song Freddie Mercury sings this phrase, but the correct way to write wouldn't be "You say you will if you could but you can't"?
You say you will if you could but you can't
This is a strange kind of reported speech: if we think about what was actually said and add some quotation marks, it might be:
You say "I will if I can", but you can't - the second person will try to do it
You say "I would if I could, but I can't" - the second person will not even try to do it, because they don't think they can
So, there are two possible correct options for the strange-reported-speech version:
You say you will if you can, but you can't.
You say you would if you could, but you can't.