If a work, such as a website, does not include page numbers, then omit this portion of the in-text citation.
Marx used “class” in “two different ways” (Calvert).
Some sources employ location indicators other than page numbers. For example, an ebook may include a numbering system different than page numbers. If your work is divided into stable sections, those sections may be cited.
Marx used “class” in “two different ways” (Calvert, par. 4).