"We have a project for a new website."
Translation:Ni havas projekton por nova retejo.
I used "retpaĝo" and was marked wrong. Am I confused or are the two words interchangeable?
You're confused - webpage and website are not exactly synonyms. A webpage is a single page you might be viewing in your browser (for example, the Wikipedia entry on Esperanto), whereas a website is a collection of such pages served as a logical whole (for example, Wikipedia).
The terminology is indeed a tad confusing...