Translation:She steams a sweet potato.
"The potato steams" has the potato as the subject of the verb. The 를 particle means the word 고구마 is the object of the verb, leaving the subject to be implied. From the looks of the comments, there should be a wider range of acceptable answers suchs as "I/You/He/She steam(s) a sweet potato" but saying "A sweet potato steams" doesn't match the grammar.
Not really. It's extremely common in every-day Korean to just exclude the subject from sentences because they're implied in the conversation. So the subject of this sentence could literally be anything since there is no context. I/You/He/She/It/You/You all/They/My dog/Mom/Dad/Kevin/My lawyer .... literally anything could technically be the subject of this sentence. However, given this is DuoLingo, I think at least all basic pronouns should be acceptable answers as the subject.
Durolingo's answer on the Test: "we steam the sweet potatoes." Note the Durolingo answer above "She steams a sweet potato." Some one at Durolingo needs to explain the changes in Durolingo's change in usage of pronouns. Also, need to explain the change in plurality of the word potato versus potatoes. Things like this waste my time, and probably others time. I have visited this problem before. I have copied Duroling's answer and will cut and paste it on my THIRD attempt. If it comes up wrong again ... then I quit as I have lost enough study time due to Durolingo's mistakes.