"He appeared without a shirt."

Translation: Ele apareceu sem camisa.

[Lingodingle]

Would using the article "uma" before "camisa" be wrong. Is there any rule for not using it here"

[Paulenrique]

That looks weird and wouldnt keep the same meaning. You would use thay if you added some more information... (ele apareceu sem uma camisa para vender) but then, that wouldnt mean who was wearing no shirt.

[michelwander]

It is right, but it kind of sounds weird. But I definitely think it should be accepted as an answer.

[BobKaucher]

This represents a serious limitation with this methodology of teaching. The English version would be used in both scenarios mentioned by Paulenrique but it would not be a good idea for Duolinguo to accept Lingidingle's answer because it is situationally specific and represents a shade of meaning not carried by the English version in normal conversation. While Lingodingle's translation was grammatically correct and possible, it does not convey the same meaning in every-day speech.