The AT symbol doesn't work because you have other keyboard layouts installed
Remove all but the USA keyboard layout and the @ symbol works again. My mother had the spanish keyboard layout installed, once we removed it viola. Mom is happy again.
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Every keyboard has an @ somewhere, otherwise you could not write any emails. You just need to look for it.
https://support.apple.com/guide/mac-help/use-the-keyboard-viewer-on-mac-mchlp1015/10.15/mac/10.15
On a Spanish keyboard the @ is produced via Option 2.