When Steve and Tony meet for the first time aboard the helicarrier, they already hold preconceived notions about each other. Tony feels resentment over the way his father, Howard, compared him to Steve, and Steve assumes that Tony is selfish and unwilling to make hard sacrifices.

While the pair did meet shortly after the snap, Tony was still reeling from the loss especially with Peter Parker (Tom Holland) gone that he didn't have the emotional capability to make peace with Steve.

Kind of a minor piece of dialogue in the film, but it seemed clear, even to Tony post-battle on Titan, that Cap also tried his best to stop Thanos in Wakanda and failed. So why is Tony still so angry with Steve in the beginning of Endgame?

Even though they were both part of the MCU's Avengers, there are reasons to believe Tony Stark (Iron Man) and Steve Rogers (Cap) weren't real friends.

It completely depends on what angle you’re looking at it from. If you wanted to bring real life politics into the equation then Tony would be right, because a group like the Avengers needs to work under jurisdiction. In universe, however, Steve is 100 % right, the movie explicitly agrees with Steve.