Re: 404 errors on all links
Firstly It’s better if people didn’t piggyback (hijack / resurrect) old threads, it appears to make sense when one finds a thread that mirrors ones own issue but is not generally considered best practice.
Secondly scroll back up to mercims post and the link to the sort of detail you might need to provide the community in order that accurate help may be offered.
To the issue it strikes me that you haven’t possibly? completed all steps to enable MS (Multi Sites) if the answer to that is ‘Not sure’ then likely you haven’t and it may be wise to read through the steps outlined in the WP codex:
https://codex.wordpress.org/Create_A_Network
I notice that your site is running as a sub domain this will likely be an issue if trying to run blogs as sub domains.
You might want to check that you have and can write to the .htaccess file (can you change the permalink structure from the backend and have it report success or a message that it can’t update the file)
