"In my opinion this dress is not beautiful."
Translation:Bana göre bu elbise güzel değil.
If you answered "Bence bu elbise güzel değil", this should ALSO be accepted as a correct answer so if its not accepted, just report it.
However "Bence bu elbise güzel değildir" has a slightly different meaning than "Bence bu elbise güzel değil".
Consider you are picking a wedding dress with your friend from a catalogue. If you point a dress and say "Bence bu elbise güzel değil", you are saying "In my opinion this dress is not beautiful.". In this example you KNOW and DECIDED that the dress is not beatiful even though you haven't seen it yet.
However if you say "Bence bu elbise güzel değildir", its meaning would be more close to "In my opinion this dress might not be beautiful.". In this scenario, you are not 100% sure if dress is beautiful or not yet. But you guess that its not beautiful and you believe if you see it in real life, its likely that you will not like it.
So "güzel değil" indicates a certainty, where "güzel değildir" indicates a guess or belief.