English Translation
You haven’t a care in the world.
I asked http://www.fundeu.es/ about this sentence and got this response:
Son adecuadas No tienes nada por que preocuparte o No tienes nada por lo
que preocuparte
So I think the only instance where you can use an accented qué in a sentence like this is when it doesn’t refer back to anything: “No tienes por qué preocuparte”.