The requested URL / was not found on this server. Error 404, what is it?

The HTTP 404 Not Found Error means that the webpage you were trying to reach could not be found on the server. It is a Client-side Error which means that either the page has been removed or moved and the URL was not changed accordingly, or that you typed in the URL incorrectly.

This is one of the kind of error for HTTP status code 404.
It can be viewed in any one form as
"404 Error"
"404 Not Found"
"Error 404"
"The requested URL [URL] was not found on this server"
"HTTP 404"
"Error 404 Not Found"
"404 File or Directory Not Found"
"HTTP 404 Not Found"
"404 Page Not Found"
From above context, it is clear that either the given website is not found that means

1. User might have wrongly typed the website URL
2. Requested Page is temporarily or completely removed from website or moved to some other website.
3. Mostly users redirect to an error page saying the above reason or redirect to a correct page in case if the website is quite active.
How To Fix the 404 Not Found Error i.e. The requested URL [URL] was not found on this server
Retry the web page by pressing F5, clicking the refresh/reload button, or trying the URL from the address bar again.
The 404 Not Found error might appear for several reasons even though no real issue exists, so sometimes a simple refresh will often load the page you were looking for.
Check for errors in the URL. Often times the 404 Not Found error appears because the URL was typed wrong or the link that was clicked on points to the wrong URL.
Move up one directory level at a time in the URL until you find something.
For example, if www.web.com/a/b/c.htm gave you the 404 Not Found error, move up to www.web.com/a/b/. If you get nothing here (or an error), move up to www.web.com/a/. This should lead you toward what you're looking for or at least confirm that it's no longer available.
Tip: If you have moved all the way up to the website's homepage, try to run a search for the information you're looking for. If the site doesn't have a search function, try navigating to the page you want using category links to dig deeper into the site.
Search for the page at a popular search engine. It's possible that you simply have the entirely wrong URL in which case a quick Google or Bing search should get you where you want to go.
If you do find the page you were after, update your bookmark or favorite to avoid the HTTP 404 error in the future.
Clear your browser's cache if you have any indication that the 404 Not Found message might just be yours. For example, if you can reach the URL from your phone but not from your tablet, clearing the cache on your tablet's browser might help.
You might also consider clearing your browser's cookies or at least the one(s) involved with the website in question if clearing the cache didn't work.
Change the DNS servers used by your computer but usually only if an entire website is giving you a 404 error, especially if the website is available to those on other networks (e.g. your mobile phone network or a friend in another city).
404's on an entire website isn't particularly common unless your ISP or government filters/censors websites. No matter the reason, if it does happen, giving another set of DNS servers a try is a good step to take.
Finally, if all else fails, contact the website directly. If they've removed the page you're after then the 404 error is completely legitimate and they should be able to tell you that. If they've moved the page and are generating 404's instead of redirecting visitors to the new page, they'll be happy to hear from you so they can go fix it.
Hope that helps!!!

It has nothing to do with any nonsense above. That's just copied and paste garbage from people who knew a guy who heard a guy who thought a guy knew something...

You have a simple forwarding problem.

Either http to https, www. to non-www, or something in between.

If you have your forwards done right, problem solved.

If you have no idea what I am talking about....get someone to fix it. Simple problem.

Since you are asking, I assume you have not even the slightest knowledge on how to fix it.

Paul