$.scrollify.next() is not a function?

I have succesfully implemented scrollify and when scrolling with the mousewheel it works fine. Except when i tried implementing a "next" button it says "$.scrollify.next() is not a function" while on the scrollify website it is listed as scrolling to the next section. https://projects.lukehaas.me/scrollify/#methods

If it helps some more info on my project: I am working in laravel 5.6 I use firefox Php 7.2 Jquery 3.3.1

Here is my html code:

<section class="scrollclass">
  <form action="/action_page.php" method="get">


    <div class="vraag test werk_container colorpad" data-section-name="werk_container">
      <div class="inmiddenvraag">
        <img onclick="scrollup()" src="" class="pijlup">
        <p>heeft u last van hoofdpijn?</p>
        <img onclick="scrolldown()" src="" class="pijldown"> 
      </div>
    </div>

    <div class="vraag test werk_container colorpad" data-section-name="werk_container">
      <div class="inmiddenvraag">
        <img onclick="scrollup()" src="" class="pijlup">
        <p>bent u vaak moe?</p>
        <img onclick="scrolldown()" src="" class="pijldown">
      </div>
    </div>

    <div class="vraag test werk_container colorpad" data-section-name="werk_container">
      <div class="inmiddenvraag">
        <img onclick="scrollup()" src="" class="pijlup">
        <p>eet u vaak brood?</p>
        <img onclick="scrolldown()" src="" class="pijldown">
      </div>
    </div>
  </form>
</section>

and my js:

function scrolldown(){
  $.scrollify.next();
}

function scrollup(){
  $.scrollify.previous();
}

Can anyone help me getting $.scrollify.next() to work?

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