Translation:My dad is a writer who writes books about butterflies.
Dobranoc na cziemnosć.
(OK, that's the best I could come up with for "A good night for mothing" :P )
Why I cannot write "My dad is THE writer who writes books about butterfiles" ? Because we have recognized him as the writer ?
I believe that using a definite article should be accepted. Are you sure that that was the reason your sentence got rejected?