Courtois made gestures of despair against himself because the goal had come from a failure in a clearance of his. The Ajax that dominated the entire first half, which threw Madrid back, finally found the goal in a rebounding play. He shot a shot and the whole stadium celebrated.
But the referee listened to the guy who was told from the VOR, the analysis room of the VAR, which is released in the Champions in this tie. They told him to look at it, to see it on video. The referee Skomina went to see him and decided it had not been a legal goal.
A player of Ajax is between the trajectory of the ball and Courtois, offside, and is part of the play because it interrupts the Madrid goalkeeper. Goal canceled First appearance of the VAR in Champions.