This can be caused by a typo in the url (address), an old bookmark in the wrong format or an attempt to reach a page that does not exist on the current site.

This may well be related to the upgrade of the site for the 10th Edition of A First Look at Communication Theory in August 2018. Most significantly:

• The site now supports only Editions 9 and 10 of the text.
• Even when referencing Edition 9, the form of the url can cause errors. The site now uses hyphens instead of underscores (edition-9, not edition_9). This change was made to avoid misreading a url when viewed as a link—an underscore can be masked by an underlined link, while a hyphen remains visible and is now the standard filler for spaces between words in the url.
• The updated website no longer has separate "book" and "instructors" home pages, but a single "home" page (referenced simply as www.afirstlook.com or www.afirslook.com/home).

Find the page you want using the menu or links on pages of the current site. Reset your bookmarks and links if necessary.

*Since the 8th edition, the site has used Search Engine Optimized (SEO) urls, which are based on a series of keywords separated by slashes, instead of the more obscure query string. These are easier for you to understand, but more importantly, search engines read them as keywords to improve site indexing.