I am receiving the "When WebCite tried to archive the page, it received a Page Not Found error from the website concerned." message for a website I am trying to archive.

Please see below, as detailed on the WebCite FAQ page: 

Why are some pages not archived successfully?

A page may not be archived for a number of reasons. The page owner may specifically prohibit archiving of their content through no-cache / no-archive tags, or via a robot exclusion policy on their site. The content may be inaccessible from the WebCite^® network (this is particularly likely if you are attempting to access subscription based content which your institution subscribes to on its users' behalf). Also, the content may be unreadable by the WebCite^® archiver (complex JavaScript based pages, or ones involving browser checks sometimes cause our archive engine to fail).

JMIR authors citing webpages, PDF reports available online, and other web-based references used to be asked to add a WebCite link to their references; however, since December 2018 this is no longer mandatory due to WebCite phasing out its service to the community. If you receive this message and you are a JMIR author, proceed without the WebCite link.