Links in a subdomain?

Question

HTML

Hello.
There is a subdomain for example - test.firma.com
A couple of css and html files are thrown into this subdomain.
The menu structure looks like this:

<div id="sidebar-wrapper">
            <ul class="sidebar-nav">
              <li class="sidebar-brand">
                  <a href="index.html">
                      Menu
                  </a>
              </li>
              <li>
                  <a href="win.HTML">test1</a>
              </li>
              <li>
                  <a href="lin.HTML">test2</a>
              </li>
              <li>
                  <a href="ast.HTML">test3</a>
              </li>
              <li>
                  <a href="trav.HTML">test4</a>
              </li>
              <li>
                  <a href="oth.HTML">test5</a>
              </li>
              <li>
                  <a href="firma.com">startpage</a>
              </li>
            </ul>
        </div>

The files are all in the test folder.
But when you click on the link from the menu, it says that there are no such files, tell me what the problem is.
PS!!!
Everything works as written above, only 1 but!
I still have no idea what it could be.

Answer 1

provide the exact link, for examplehttp://test.firma.com/firma.com