Translation:I have not seen myself in the mirror today.
Often, Portuguese (and Spanish, if I remember correctly) will use a reflexive pronoun that will not necessarily carry over into an English translation. We would not say "I didn't look at me in the mirror" even though that's how it flows in Portuguese. Some acceptable translations: I didn't look at the mirror... I didn't look into the mirror... Duolingo marked that second one wrong, so I reported it to them. (It is incorrect grammar to say, "I didn't look in the mirror" unless you were physically inside the mirror when you looked.)
"I did not look myself in the mirror today" can have a slightly different connotation than "I did not look AT myself in the mirror today". If you can translate those two sentences the same way in Portuguese and would be forced to understand the true meaning by context, then aren't both possible translations?