Error 404 means that the link you clicked on is pointing towards something that isn't there. Although it can be a browser caching issue if you have downloaded it previously and the file name has since changed such that it is trying to download the file with the old name, it is usually something that has to be fixed by however maintains the website. You can try refreshing the page then clearing your browser's cache/cookies or even a different browser but if none of that works, you'll need to contact GOG about it.
Having worked in customer/tech support in the past, the odds are extremely low that if you contact GOG support that you'll get someone who knows what website error codes actually mean. It's not their job to fix any part of the website when it isn't working as it is well above their pay grade, and they don't have the access to do it either.
You'll need to explain the issue to the without expecting them to fix the website. I would say something like:
"Hello. I am getting an Error 404 when attempting to download the quickstart guide for the game Legends of Murder Collection, which means that the link to this file is broken either due to the file being missing or the URL pointing to it being incorrect. Would you be able to email this file directly to me at (your_email_address_here) or send me a direct link to this file so that I may download it?"
More information on Error 404:
https://www.allconnect.com/blog/404-page-not-found-error-explained https://en.wikipedia.org/wiki/HTTP_404 If you have GOG Galaxy installed, you can go to the game in your library, click on extras and try downloading it from there. If this doesn't work, let me know - I'll see if I can find the quickstart floating about the Internet.
