Phil Foden's brace had City 2-0 ahead before the Bees rescued a point via late goals from Yoane Wissa and Christian Norgaard.
Guardiola said afterwards: "We're a bit disappointed but at 2-0 up we didn't manage to hold on to the lead and unfortunately we couldn't close the game. Today we didn't take the right decisions in the final third. I had the feeling that we didn't take the right decisions in the final third to close the game.
"With the absence of the central defenders, Rodri, with many things, we are not (at our best) and we didn't take the right decisions. It's fine, it's not a good place to come. Unfortunately, we couldn't win."
On why his side are struggling to hold on to leads, he added: "I don't know, it can happen. Sometimes it happens. We have to manage a little bit better but today was not bad."