The specifics of your question are a little hard to answer. Part of the difficulty is that I'm not (and few would be) capable of giving you a blow by blow account of what Orwell got wrong or right about his descriptions of the events of May 1937. Needless to say, he definitely gets things wrong - he lost many of his notes (his diary was allegedly confiscated by communist police), and more fundamentally he was in a confused and fluid situation that he didn't fully understand. Orwell himself is fairly open about the extent to which his perspective is a limited one. If you're interested in evaluations of his wider point about the nature of the conflict, you may want to check out my own answer here, or an earlier one from u/Domini_canes.

For me at least, a slightly more interesting question (I'm doing the classic AskHistorians trick of redefining your query to something I can answer!) is why the accuracy of Orwell's account is such a big deal. Orwell has some very prominent critics, most notably Paul Preston, who is likely the most influential historian of the period working today. Preston has attacked Orwell in a variety of public and scholarly settings. This is part of a broader pattern of attempts to discredit Orwell's version of events, generally from those who are very sympathetic to the Spanish Republic. I think the issue at hand is that while any number of conservative figures have attacked the Republic's legitimacy, Orwell is a respected voice of the left, and his criticisms represent a much more potent threat to the Republic's historical memory, and have therefore attracted quite disproportionate criticism.

From my perspective though, singling out Orwell’s book as being especially flawed or biased seems to miss the point – it was written as a reaction to his immediate experiences in Spain, with all the acknowledged passion and limited scope that a personal account implies (and Orwell himself wasn’t shy about admitting that his account was constrained by such factors). In other words, this is a primary source, not a scholarly history of the conflict. This makes ‘inaccurate’ a curious label to apply to Orwell’s work. As every historian knows, any primary source contains bias, especially those describing so emotive an event as the Spanish Civil War. In the numerous primary sources I’ve considered for my own work, there’s not one that is unbiased or neutral, or able to fully escape the limitations of the author’s viewpoint. Many contain factual or interpretive ‘errors’ on the same magnitude as Orwell. That’s fine – that’s why historians try and find a range of perspectives and sources to build their case. If we dismissed each source because of its flaws, we’d soon have little to work from.

Preston’s criticism is of course much more nuanced than this. As he noted, the problem is that for many readers, Homage is the only text on the Spanish Civil War they will ever engage with, amplifying the gravity of the book’s flaws. This is a problem, although hardly unique to the Spanish Civil War. How many people have had their knowledge of major historical events shaped by literature or film? Academic historians could doubtless poke holes in most such accounts, yet it’s unarguably true that for most people, reading a scholarly monograph is hardly the way they like to expose themselves to new ideas. The best we can hope is that those who find themselves moved by Orwell’s work are encouraged to find out more about the conflict and seek out other perspectives.

An unspoken part of this problem is that Orwell was a very good writer, with a style that comes across as plainspoken and honest. It’s not just that Orwell wrote unpalatable things about the Spanish Republic, it’s that they sound so immensely convincing and Orwell himself consistently comes across as a confused yet honest witness. It is this appearance of bluff honesty which, I think, annoys his critics so – and encourages them to dig into his character and fact check his work so closely. Even so, I don’t believe that the solution is an Orwellian hatchet job simply because he failed to meet impossible standards for any writer discussing events as complex as the Spanish Civil War. Orwell (like just about any living or historical figure) cannot stand that level of scrutiny, and nor should he have to – and we should treat Homage to Catalonia as we would any primary source: critically yet constructively.