including external js library showing error in console in magento 2

Maybe you can try to include your js using require.config this way:

Create a new phtml file with this content:

<script>
   require.config({
       map: {
           '*': {
               'magnific-popup':  'https://cdnjs.....'
           }
       }
   });
</script>

Whenever your phtml is executed, above code will make an entry to requirejs-config.js file.

Then you can call that js in any other js file using define:

define(
    [
        'jquery',
        'magnific-popup' // here the js file is mapped
    ],
    function (
        $,
        popup // you can refer this object to use magnific-popup functions
    ) {
        'use strict';

     // js logic
});

In this way your js file will not be included in every page of the site but only in pages you really need it.

Hope it helps :)