True but its not saying it like how you are. You can have a bedsheet or you can have a blanket. It shouldn't have marked it wrong or else never gave us the option to use it and when you click on the word in Spanish it gives you the English words for so it is a suggestion but also an option.
Macy's sells bedsheets. The Merriam-Webstrr DIctionary defines bedsheet. Wiktionary has besheet. Dictionary.com has bedsheet. You can get a picture of a bedsheet at Stock Photos. At sentence yourdictionary.com it tells how to use bedsheet in a sentence. At vocabulix.com they translate bedsheet into English. Bloomberg tells how ro buy a bedsheet. A book you can buy on Google Books is entitled, Lincoln's Bedsheet. New York Times article. A Bedsheet Built For Two. News article, St. Louis bedsheet escapee convicted of underlying crime. Observer Reporter: Inmate found hanging from bedsheet dies. Babylon.com trranslates bedsheet into English.
And it goes on and on. However, like on duoLingo there can be found on the Net lots of usages of "bed sheet" too. Two words. The main distinction sppears to be one of class where bedsheet is the more refined and bed sheet is low brow.