Jesus said that eventually the disciples would be unable to be with Jesus, but Simon Peter still asked Jesus where Jesus was going. But Jesus reminded Peter that Jesus would eventually go to a place where Peter could not be at that time. The crucifixion was a solo activity done by Jesus. Though Peter would follow Jesus at some point afterward (13:36), presumably since Peter has accepted Jesus.
But Peter was insistent on sticking with Jesus in person! In fact, Peter proclaimed that he would lay down his own life. It's implied that Peter has a sense of pride when it comes to sticking with Jesus. However, Jesus prophesized that Peter would actually deny Jesus three times before the rooster crows (13:38). Jesus knew that likely from the effect of separation (and the current atmosphere in the moments before the crucifixion) as well as Peter's heart. From the instances of the denying itself, it's likely that Peter has a fear problem and could not contend with being separate from Jesus in person, especially when Jesus was on the cross.
However, even though Jesus foreknows Peter's denials from Peter's heart, Jesus is not done with Peter.