Hola Elias

I'm not sure which test you're referring to but I imagine you're talking about this question you got wrong, when asking for "un" hermano. 

Tengo __ hermano.

I have a brother.

The correct answer is "un", not "uno". For "a ..." before a masculine noun you need "un" as explained in the lesson.

I hope it clarified it.

Saludos