Why can't we write "vandaag is het slecht weer" ?
It's better to stay close to the original sentence whenever possible, so: Het weer is slecht vandaag.
But that answer was not accepted by Duolingo, not yet.
Actually it's been in the accepted answers list for a long time (the sentence wasn't edited recently). Are you sure it marked you wrong without any other mistakes?
sorry my mistake it was a slightly other sentence. Same words but in another order. Something like that. Het weer is vandaag slecht. But i reported it. And you accepted it :)