Spanish speakers don't use an indefinite article before an unmodified noun when it refers to a concept or idea of something, rather than a particular thing. See below how autor is not preceded by un because it is not modified, whereas profesora is preceded by una because profesora is modified by the adjective joven.

I am a writer.	Soy autor.
She is a young professor.	Ella es una profesora joven.
I don't have a fatherland.	No tengo patria.
I'm wearing a shirt.	Llevo camisa.

We also don't use an indefinite article after the bold words in the table below.

We arrive in half an hour.	Llegamos en media hora.
He writes with a pencil.	Escribe con lápiz.
We travel without a car.	Viajamos sin coche.
A certain person is coming tonight.	Cierta persona viene esta noche.
in such a case	en tal caso
What a pity!	¡Qué lástima!

This might seem like a lot to learn, and there will certainly be other cases we have not mentioned here. There is no reason to feel frustrated though, since you will develop an intuition for this soon enough.

Usages of comprar