Nonstandard but not incorrect. Besides, we're learning Portuguese here, not English.
The hint for 'sem' offers 'with no' here as well, so perhaps it can also be interpreted as 'I sleep with no pillow'.
I sleep without pillow marked as wrong. This is confusing! For many answers when I write "a" or "the" still it marks as Incorrect. Similarly one query: Why the question does not have "Eu durmo sem um travesseiro" Any suggestions?
If you're saying "I sleep without pillow" is correct in English, that is not correct. Either you have to either add an article (direct or indirect) before the word pillow or make pillow plural so the sentence reads "I sleep without pillows."
The question I have is why don't you need an article before pillow in Portuguese, so the sentence reads "Eu durmo sem o/um travesseiro"?
there are times when the article isn't required in portuguese and is understood. putting the aritcle does not make it incorrect in portuguese but in terms of usage it is apparently uncommon. I am yet to master when exactly the article should be avoided