It is hard to say, because if I use your link, it will show my collection and if I go to your profile and check out your 1991 Pro Set, I'll see each card you have without any missing gap. The only solution I see is to go yourself in your profile, take your 1991 Pro Set collection one by one and look for the missing number. I would check the special collectible cards in the list first, it is plausible that it is one of them.
SET BUILDER: Baseball ◉ Hockey ◉ Misc. Sports ◉ Non-Sport & Oddballs
PC: Felipe Alou ◉ Derek Aucoin ◉ Steve Begin ◉ Jennifer Botterill ◉ Kevin Dineen
L.D.T. ◉ Bob Kudelski ◉ Manon Rheaume ◉ Gabriela Sabatini ◉ P.J. Stock