It's not that white women aren't allowed write Latino characters. They absolutely can.
Jeanine Cummins did not write Latino characters. She wrote harmful stereotypes and absurd scenarios. Her knowledge of Mexico is so bad it's like the only research she did was watch Coco with her children twice and thought, yes, that's good enough. Her best scenes she cribbed from other, better writers, like Luis Alberto Urrea. The book is getting dragged, rightfully so, but that's it. Take your criticism and go.