Free will cannot be an illusion given that it is a feeling that dwells within each and every human being. If i can try to differentiate the two is that an illusion is something or a thought that someone has come up with in their minds whether it is wrong or right, and they try to make it as real as it can be, while free will is when someone does something without been asked or urged to do. let's give an example of someone deciding to go and volunteer to take care of the old people in an old age facility for one month without being paid a cent, that is free will. they did it out of their own volition no one pushed them to do it. so i cannot say that an illusion is freewill those are two different things. Illusions may come to mind sometimes, and you start having or fantasizing over something and you start seeing yourself in that scenario, even when you know it is impossible, like when we would drool over celebrities homes, and daydream about being there, and having barbecue with them when we know it would take a miracle for that to happen. Now that is what i call an illusion. Thinking of the impossibilities, but free will you actually get do do what you are thinking with or without help.