"I do not want a shirt because I want a skirt."

Translation: Eu não quero uma camisa porque eu quero uma saia.

[lalaan96]

can't i use "pois" instead of "porque"??

[melesana]

Agreed, Pois seems to make as least as much sense here.

[LiClemente]

You could, it was a mistake they didn't accepted. (BTW, if you use "pois" it has a "," before, always. ;)

[Gilson.Oliveira.]

I am a native brazilian portuguese speaker, and in this case is not wrong use "pois" instead of "porque".