What's wrong with sticking "vannak" on the end? Is it wrong or just not necessary ?
It's wrong. However, you can say: Vannak fiatal művészek akik szépek. But it means: There are young artists who are pretty.
Is "A fiatal müveszek nagyon szépek" wrong?
Save for the proper lengthening of the vowels in "művészek" you also added a nagyon which isn't asked for in the English sentence. (There's no 'very' there.)