I think there's a few factors for why he hasn't achieved European success at City, luck is absolutely one of them as all European success involve some modicum of luck, but I think the other big reason is the way they approach the game. While it's obviously delivered them brilliant league results, I think it's worked against them at key moments in European ties.
The way City play is incredibly hard to achieve. Every facet of their game is meticulously organised, both in terms of what they do on the ball and where they should be off it. They stringently pass it out from the back, but eschew the mindless back and forth between defenders that many other sides employ and instead have genuine progressive minded thinking when playing it out, as well as their defenders often being found very high up the pitch it also involves a lot of coordinated movement from their other players, constantly moving and offering different outlets to keep the ball progressing, rarely sharing the same vertical corridor, and there'll always be players in key positions out wide for the switch to stop the approach being too uniform. Having no traditional striker allows them to be positionally fluid in a way that makes defending against them very hard, and while they're very good at ball retention it's not without purpose, very few sides can create as many chances as this City side. Defensively playing so high and with a very drilled and precise press they suffocate the opposition after losing the ball, and even if they don't manage to regain it the way they stagger their defensive lines makes it very hard to play through them, and trying to exploit the high line with a long ball is difficult as Walker is so adept at countering this.
All of this leads to a site that plays great football and is a nightmare to play against, and the league format lends itself to a comfort zone in which the balance of probabilities means City will usually win, and mistakes and losses can be rectified over the course of the 38 games. In Europe at certain key moments I think this City side have been rattled. And it's not an indictment of the players, maintaining such consistency in the league season after season shows mentality. The way they play is so meticulous though, involving precise decision making and composure and set up in a way that mistakes can be heavily punished. Once the situation deteriorates it's very difficult to try and maintain the same approach with the same confidence, they become slightly more hesitant, decision making starts slipping slightly and the urge to deviate from their usual style and revert to something less risky, if ultimately less potent will be hard to resist. Madrid were out for the count, they looked aimless towards the end of the leg, Camavinga and Rodrygo added impetus and two sucker punch goals and you could see the confidence from this City side dissipate.
With his Barcelona side, I think the main two reasons that they had the European success that this City side hasn't yet is that the Barcelona side Guardiola moulded are the best in modern times and could dominate games like no other side I've seen and legendary players. The other is that football sides are a lot more adept at playing Guardiola sides. He changes and tweaks his approach often, the way his City side play now is pretty different to City of a couple seasons ago, but many of the underlying philosophies are still there.
