I lookcked at the sentence again and I think Duo rejected becauseI thought that potato is feminine like in all the other latin derived; la patata, la papa, a batata, la pomme de terre. This beg the question why is Romenian different?
I think papa, patata, batata are all related, while "cartof" may be from another family ;)
However, if you want the potato to be feminine you can use the word "barabulă".
O barabulă - două barabuli. Don't expect Duo to have heard of it though... ;))