"There is nothing more to say."
Translation:Il n'y a rien de plus à dire.
Do you mean why isn't it "Il n'y a rien de plus à dire"? If so, then no, it is grammatically incorrect.
"ne ...rien" is required to be placed around the pronoun (if there is one) and the verb (in this case y and a).
However, Rien can be used alone as a response:
"Qu'est-ce que tu fais ?"/ "Rien"
"What are you doing?" / "Nothing."
I think what he meant is why isn't it "il n'y a rien plus à dire" which would also have been my question...
It is a set expression → rien de plus = nothing more, rien de moins = nothing less, etc.
il n' y a rien plus a dire should be acceptable. or do you really need the rien de plus?
Why is the accented "a" required before dire which already means "to say"? This makes it nothing more to to say.