"Họ không biết dùng nó."

Translation: They do not know how to use it.

[JackyDW]

Does "biết" mean both "know" and "know how"?

[LanguageButcher]

"biết" = "have knowledge of"

So yes, it means both "know" and "know how."

[Eric400471]

"They don't know how to use it" is marked wrong.

[Huy.B]

"They do not know the use of it"
Is this correct?

[Mu6OVm]

I don't think so.

You can know the use of a plane, does not mean that you know how to use it!