"I have a laptop."
Translation:Mae gen i gliniadur.
Please remember that sentences can have multiple alternatives, and we can't see from the sentence discussion which alternative you may have been shown -- only the one marked as "best", which is currently Mae gen i gliniadur.
Typically, the alternative that is closest to what you wrote is shown to you if you answer "incorrectly", which may be quite different from the "best" alternative in cases. And we can't see what you wrote, either.
So when asking whether something was incorrectly marked wrong, it's best to say explicitly what you wrote and/or the sentence that Duo showed you.
(If, on the other hand, you are sure that you were correct, then please just report it. Especially on the Welsh course, reports get acted on fairly quickly in my experience, and then there's no point in leaving a comment on the sentence discussion that is obsolete in a couple of days' time.)