En-words that end in a vowel get just -n in the definite form, that's why soppa is soppan here.
Well make sure not to call a Swede that, because it means "the piece of garbage" ;-)
I wouldn't rule that out as a bug, but I've been ignoring case in the apps for a few years and that's never caused my answer to be marked wrong.
I entred correct 'Soppan', than it says it's not correct and is giving me the correction 'Soppan'... I can't understand the mistake.
It's a Duolingo bug, it will rarely but occasionally say that the correct answer is wrong. It is very annoying.