Library name should lower case (not Capital Case) and Library should be installed via Composer

For anyone stumbling on this:

Until this is fixed someday, you can override the file path(s) of the library in your theme's info.yml:

libraries-override:
    fiu/magnific:
    js:
      /libraries/Magnific-Popup/dist/jquery.magnific-popup.min.js: /libraries/magnific-popup/dist/jquery.magnific-popup.min.js
    css:
      theme:
        /libraries/Magnific-Popup/dist/magnific-popup.css: /libraries/magnific-popup/dist/magnific-popup.css

Reference: https://www.drupal.org/node/2497313 →

maxator → made their first commit to this issue’s fork.

maxator → changed the visibility of the branch 3.x to hidden.

maxator → changed the visibility of the branch 3.x to active.