Address problems?

Question

GitHub

There is an address:
my-site.github.io/ - so for example (in fact, there is no such thing, but this type is created on github pages)
created in github pages
everything is fine and you can even see the repository file, for example:
my- site.github.io/stylesheets/print.css
- this file was originally created from the github pages template
, but the fact is that when I add my folders and files myself, for some reason I get errors and I can’t go through such a direct link, for example. added my folder and the file in it to the root of the my-site.github.io repository: css/mycss.css, but I can’t follow the link like this:
my-site.github.io/css/mycss.css
(
what to do?
maybe is it related to opening access to the file?and how to open access to it?