I do think any Democratic candidate would have really tough time, but probably the biggest mistakes ...
Not emphasizing enough that the Republicans wanted to not only deport illegal immigrants but reduce legal immigration (while they were for both moderate, reasonable enforcement and legalization like Parole in Place, legalization if been here for 10 years and married to US Citizen).
Maybe not emphasizing enough that inflation was global. And/or probably not be so absolutist against tariffs, so therefore implicitly for complete free trade, not realize, empathize enough with why a lot of people really dislike free trade policies/foreign competition and therefore like at least idea of tariffs. Or have alternate ideas, appeal for how to increase manufacturing.
Probably a lot less but Harris not going on Joe Rogan when Trump did was still pretty bad/big, her doing so would have at least shown bravery and in it she could have blasted Trump more, more effectively, for him not being willing to do more debates.