When you deploy to Github Pages, your site typically lives at username.github.io/repository-name/, not at the root. Using ../ tries to go up from your current path which can break depending on where the page is located.

Replace repository-name with your actual repo name:

<div class="nav-links">
    <span class="material-symbols-outlined" id="closeMenu">close</span>
    <a href="/repository-name/Welcome/index.html" class="navurl navpart">Welcome</a>
    <a href="/repository-name/Overview/overview.html" class="navurl navpart">Overview of Experiment</a>
    <a href="/repository-name/Simulation/simulation.html" class="navurl navpart">Simulation</a>
    <a href="/repository-name/Photos/photos.html" class="navurl navpart">Photos</a>
</div>