Refusing to lean into and push progressive policies that are broadly popular amongst the electorate, as Obama did in 2008, and instead attempting to appear more right wing to court an imaginary disaffected Trump voter bloc. She failed to offer any compelling vision of change and it cost her dearly.
The whole "Republicans for Harris" bullshit dog and pony show, along with the very conservative messaging and rhetoric that the campaign adopted starting with the DNC in August almost certainly alienated a good chunk of (or at least disillusioned) younger and/or low-propensity voters. Those two groups are very demonstrably angry with the status quo, an d nothing says "status quo" like an establishment Democrat taking a public right turn and palling around with Liz fucking Cheney. It certainly made me want to throw up in my mouth and I don't belong in either of those categories.
Refusal to break with the current sitting (and quite unpopular with the electorate) Joseph Robinette Brandon in any meaningful way on any current administration policy, and actually pivoting further right on some of them, such as the border/immigration. She had a TON of momentum early and this was the time to do it, to really set yourself apart and energize voters.
The absolutely baffling and inexplicable decision to send Ritchie fucking Torres of all people, and Bill Clinton, to Michigan to try and appeal to Arab-American voters was one of the dumbest things I've ever seen a campaign do in my lifetime. Not to mention, again, courting the fucking Cheneys.
Picking Tim Walz but not actually allowing him to play to his real strength as a politician, which is pushing progressive policies in rural red/purple areas of the country, and instead effectively relegating him to being the "hey look at the Republicans, aren't they weird?" guy.
Failing to actually address the economy in any meaningful way that would resonate with undecideds (who decisively broke for Trump) and low-propensity voters.
The list is fairly long, but that isn't to say she didn't do plenty of things right as well. You're not wrong that America is very racist and misogynistic. She needed to run a near perfect campaign to win and she didn't come even close to that. The signs were there all along, and pretty much everyone ignored them, to our detriment.