"You should not be sad."

[letjustin]

Is "Bạn nên không buồn" also ok?

[cooldcs]

You sentence is understandable, but not as natural.

[mua-Dong]

typo: "Your sentence"

[StewartMM]

In english, it seems more natural.. "should" is what is being negated, not "sad" but the Vietnamese construction implies the opposite.

[Thomas457134]

I put "được buồn" as I thought she was being sad in the present?

[vngdhuyen]

I think you confused "đang" and "được".

đang is the present continuous marker.
được has different uses: it could mean the verb to obtain, to get (sthg); it could be translated as to be to form the passive voice; it could also act as a particle that express ability, possibility, similarly to what "có thể" does.

[Thomas457134]

Thanks for you help vngdhuyen and yes confusing is correct