Christopher Nkunku’s chipped effort was going in until the 30-year-old, bent on guiding the ball in, took a bad touch on the goal line to see his effort hit the post.
The miss denied Thomas Tuchel’s men a chance to win the trophy at the Parc des Princes on Sunday and the Cameroon international feels sorry for his action.
"I thought Nkunku was going to give it to me - and that I thought the defender was going to get it,” Choupo-Moting said.
"I wondered if I was offside and it all happened very fast. I touched the ball, it hit the post and, well, it's a real shame because the ball was going in, Sorry about that."
Manager Tuchel also reacted to the incident, saying: "I think he's lacking confidence. He's not here to start every game but he is at the moment because we have a lot of injuries.
"It's not his role. He's a great substitute."
PSG are on 81 points from 30 games and a draw against second-placed Lille next Sunday will crown them Ligue 1 champions.