Todos sábados = Every Saturday
It's all right to switch "Every Saturday" from the end of the sentence to the beginning of the sentence, but DL usually marks unnecessary switches wrong. This being said, I would report it again if I were you, as long as I wrote "Every Saturday, we eat chicken."
Todos LOS sábados. Todos sábados is meaningless in Spanish.
Every Saturday we have chicken. It was accepted because have is a delexical verb and we have chicken is a delexical structure.
Because you aren't including the word "todos" in your translation, which means every or all.
DL tells me "You used the plural "Saturdays" here, instead of the singular "Saturday", however "Los Sabados" is plural.
Learning a new language can be challenging in many ways. Part of fluency is learning how to adjust what words we use, depending on what sounds most natural in the language we're using.
If I were a native Japanese speaker learning English and I said: "I Monday school, Japanese language study do" this would be a bad translation, even though it is a word for word match of 私は月曜日に学校で日本語を勉強します。For an English audience, I would need to say "I study Japanese at school on Mondays."
Likewise, if I were to translate "Todos los sábados comemos pollo" as "Every the Saturdays we eat chicken", though it's an exact translation of the words in the original Spanish sentence, it's not the most natural sounding sentence for an English audience. A better fit for the audience would be "Every Saturday we eat chicken."
I hope this helps explain why the lesson asked you to adjust your answer. :)