Formal, literary prohibition used at the end of a sentence. Equivalent to ～してはいけない in modern Japanese.
You must not enter here.
Rare, formal or literary expression of impossibility. In modern Japanese, ～できない is far more common.
This problem cannot be solved.
These documents must not be taken outside.
べからず is the negative classical form meaning 'must not / shall not', the direct opposite of the obligation sense of べし.
Derived from the classical auxiliary verb べし (beshi) in its negative form べからず (bekarazu), meaning 'must not' or 'cannot'. The form is now largely restricted to formal written language.