Not able to link my href to linkedin in my first project, Front End Web Developer program

Would help to see your code so we can assist you further.

This is the footer:

<footer>
      <ul>
        <li><a href="https://www.linkedin.com/in/brendac58282114b/">LinkedIn</a></li>
        <li><a href="https://github.com/brendacardona/" class="social github">Github</a></li>
      </ul>
      <p class="copyright">Copyright 2019, Brenda Cardona</p>
    </footer>

and the css

footer ul li {
  display: inline-block;
}

a.social {
  display: inline-block;
  text-indent: -9999px;
  margin-left: 5px;
  width: 30px;
  height: 30px;
  background-size: 30px 30px;
  opacity: .4;
  transition: all .5s;
}
a.linkedin {
  background-image: url(linkedin.svg);
}
a.github {
  background-image: url(github.svg);
}
a.social:hover {
  opacity: 1;
}
.clearfix {
  clear: both;
}

Your code is fine and the href value points to the exact path (your profile) and works correctly. You say that the link doesn't work if you are signed out. This is not the case, the link still works (any href value will still work regardless of changes to the page you are trying to visit), however, you need to be logged in to LinkedIn in order to see profile pages. I guess you are unaware of that, but it even tells you when you try to access a profile.

Hmm, very strange! I am on unit 5 of the same tech degree and every student should have a clear understanding of how LinkedIn works as in a later course, you are recommended/required to create a LinkedIn profile when searching/applying for jobs. Whoever reviewed your project should understand how the website works and should not have failed your work for that reason. You could ask around in the Slack community of the Front-End Tech-Degree to see what some other students and moderators say about it.