Is a noun's article always necessary

Is a noun's article always necessary
 

How weird does it sound to a Spanish speaking person if you say, for example, no bebemos durante almuerzo, instead of el almuerzo. Is it grammatically correct?

It sounds pretty Tarzanesque, or like a short note or a telegram "no beber durante almuerzo".

Are there any exceptions? The reason I ask is that Duolingo sometimes omits the article, and I wonder whether or not that is a mistake

It depends on the context. Concrete situations like having lunch cannot be expressed without the article.
Can you quote the examples so we can try explaining why those articles were omitted? :)

There are certainly times when a definite article is not used.
And there are times when a definite article is needed in Spanish but omitted in English.

There are some good sites that provide some rules.
Have a look here.

Not sure how to tag people in posts, or if I even can, but @angelicadealquezar I don't remember any of the specific examples. @Rusty thank you very much!

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