Read John 21:23 Using Other Translations
Then went this saying abroad among the brethren, that that disciple should not die: yet Jesus said not unto him, He shall not die; but,If I will that he tarry till I come, what is that to thee?
So the saying spread abroad among the brothers that this disciple was not to die; yet Jesus did not say to him that he was not to die, but, "If it is my will that he remain until I come, what is that to you?"
So the rumor spread among the community of believers that this disciple wouldn’t die. But that isn’t what Jesus said at all. He only said, “If I want him to remain alive until I return, what is that to you?”