In French, to say « Je ne dois pas travailler. » is usually understood thanks to the context. But I usually hear/use it to say: « I don’t have to work. ».
For example: “Aujourd’hui c’est dimanche, je ne dois pas travailler.”

For the other meaning, it’s usually better to say: “Je ne PEUX pas travailler.
Example. « Je ne peux pas travailler car mon médecin me l’a interdit. ».

Other sentences with the same meaning as « I don’t have to work. »

• Je n’ai pas à travailler.
• Je n’ai pas besoin de travailler.
• Je ne suis pas obligé de travailler.

There are really two forms to keep in mind as pertains to the verb devoir; notice that you do not negate modal verbs in English, whereas you do negate the corresponding verbs in French.

I

(a) devoir ne pas faire qqc: obligation de ne pas faire (être, faire, laisser, omettre, differer, tarder)

(b) ne pas devoir faire qqc: absence d’obligation de faire (être, faire)

The form you retain is “(a)”; it is not proper to express “must not do” as “must not do” is taken as “having no obligation to do”; it is the right form when “must not do” means “having the obligation not to”, which is the sense you give to “must” in “I must not work”, sense that must then correspond to A below.

• A      I must not work overtime; it’s forbidden by law. (obligation not to _ Je dois ne pas travailler en heures supplémentaires. (a) not commonly used
  Again, the form “Je ne dois pas travailler en heures supplémentaires” (b) is used, or, apparently better, “Je ne peux pas travailler en heures supplémentaires.”)

In conclusion, the form you retain is applicable in this strict sense of “must”, but it is not as commonly used as the form (b), or the better form using “pouvoir”.

II

The second meaning is then also translated by (b), but as well by other forms;

• Je ne dois pas travailler. (b) (acceptable)
• Je n’ai pas à travailler. (best)
• Je ne suis pas obligé de travailler. (best)
• Il ne faut pas que je travaille. (not too good)