1. A relative clause expressing the inability or lack of success in resting or relaxing
I know that you cannot rest until the project is finished.
Sei que não consegues descansar até o projeto estar terminado.
2. Used to describe someone who is unable to relax or find peace due to worry, stress, or other circumstances
The thing that you cannot rest about keeps you awake at night.
A coisa que não consegues descansar te mantém acordado à noite.