Why not "The fault is mine"?
Fault = culpa
"The fault is mine" = "A culpa é minha"
it is my mistake. Right?
Yes. that's the most natural way to convey the idea
My bad I used an informal expression.
I hope "It's my fault" would be accepted. I reported. :)
It's essentially the same idea, one which you can also convey with "A culpa é minha" (culpa also means "guilt/blame", so it aligns better with fault in terms of intensity).
It's Dou's mistake - when it says " It's my mistake" is wrong.
But saying "It's Dou's mistake" instead of "It's Duo's mistake" is your mistake, right? :-)
That's right but I am dyslexic.
You may not be spellbound by duo, then.
Crazy that Duo has "my bad" (I so hate that expression) in its lexicon, but can't understand so many other very common and much more elegant ways of saying this in English.
I think simply saying "my mistake" or "my bad" sounds more natural than "the error is mine", which sounds a bit strange.
is it also possible to say "o erro é de mim" or does that just sound silly?
No. You can say dele(s) (de ele(s)) or dela(s) (de ela(s)), but not de mim. another alternative is "de minha parte".