"Eu não cozinho desde a semana passada."
Tradução:I have not cooked since last week.
I've not cooked since last week. ou I have not cooked since last week.
Both are the same sentence. The first one is correctly written in this form = I haven't cooked since last week. The present perfect is used to express an action occured in the past, and that is still occurring in the present. Look at this exemple= It hasn’t stopped raining yet and the kids haven’t finished their homework! - Ambas as ações começaram anteriormente, porém, no presente continua chovendo e a tarefa escolar continua sem ter sido concluída.