'The shelf is beautiful' is a fairly rare usage: but the more commonly encountered 'great', which is offered in the drop-down options is marked incorrect. How many 'beautiful' shelves does one encounter? If 'great' (!) is wrong, then I think this is a poor choice of words.
Why not? It's probably not the shelf itself, but the things on it and how they're arranged and presented.
- This shelf of antique figurines is beautiful!
- This shelf is beautiful!
The drop down hints are not supposed to work in every sentence. You have to chose a translation that has the right meaning in both languages.
On the other hand, if it is offered as a hint, then there would have to be a good reason for it to be marked incorrect.
If the shelf can be "beautiful," I do not see why it cannot also be "great."
Of course the root problem is that the sentence offered isn't very natural, and this is always going to lead people to come up with varied translations to fit.