Learn Croatian: Nitko ne smije ići odavde dotamo. - Nobody is allowed to go from here up to there.

They are fusions of prepositions and demonstrative adverbs. The word odavde combines the preposition od (from) with the root for 'here' (ovde). Similarly, dotamo combines do (up to) with tamo (there).

Could I just say od ovdje do tamo instead of odavde dotamo?

While you might hear the separated forms in casual speech, the fused forms odavde and dotamo are the standard, most natural way to express spatial boundaries like 'from here' and 'to there' in Croatian.

Why does the sentence use ne smije instead of ne može?

The verb smijeti means 'to be allowed to' or 'to have permission', so ne smije specifically means the action is forbidden. If you used ne može, it would imply that nobody physically can go, rather than that they are strictly prohibited from doing so.

Why are there two negative words, nitko and ne, in this sentence?

Croatian requires double negation. Whenever you use a negative pronoun like nitko (nobody), the verb must also be negated with ne. English speakers often forget this rule and mistakenly try to say nitko smije, but that is grammatically incorrect in Croatian.