I saw following quotes:
アメリカは初めてです。
This is my first time to America.
I've learned about <n.> は <n.|adj.> です
. Checking at several dictionaries, '初めて' is an adverb. Is it OK to use an adverb in <n.> は <adv.> です
?
No, you cannot attach です to an adverb. But 初めて is both an adverb and no-adjective. (A no-adjective is a special type of noun that is semantically similar to an English adjective. See this question.) You can confirm this fact in jisho.org's entry. For example, you can say 初めてのアメリカ, which means "(my) first (trip to) America".
(Similar things happen all the time also in English; fast as in "this train is fast" is an adjective, but fast as in "this train runs fast" is an adverb.)