Fatal error: Class ‘WPCF7_FormTag’ not found

For some reason, I get the following error when attempting to use this plugin, even though I’m following the steps in the instructions properly:

Fatal error: Class 'WPCF7_FormTag' not found in /www/htdocs/millime/wp-content/plugins/contact-form-7-multi-step-module/form-tags/module-multistep.php on line 58

My code looks like this:

<h4>1. Uw gegevens</h4>

<div class="vc_row">
  <div class="vc_col-sm-3"><label for="naam">Voor- en achternaam</label></div>
  <div class="vc_col-sm-3">[text* naam]</div>
</div>

<div class="vc_row">
  <div class="vc_col-sm-3"><label for="naam">Straat</label></div>
  <div class="vc_col-sm-3">[text* straat]</div>
  
  <div class="vc_col-sm-3"><label for="huisnr">Huisnummer</label></div>
  <div class="vc_col-sm-3">[text* huisnr]</div>
</div>

<div class="vc_row">
  <div class="vc_col-sm-3"><label for="postcode">Postcode</label></div>
  <div class="vc_col-sm-3">[text* postcode]</div>

  <div class="vc_col-sm-3"><label for="woonplaats">Woonplaats</label></div>
  <div class="vc_col-sm-3">[text* woonplaats]</div>
</div>

<div class="vc_row">
  <div class="vc_col-sm-3"><label for="telefoonnr">Telefoonnummer</label></div>
  <div class="vc_col-sm-3">[tel* telefoonnr]</div>
</div>

<div class="vc_row">
  <div class="vc_col-sm-3"><label for="emailadres">Email</label></div>
  <div class="vc_col-sm-3">[email* emailadres]</div>
</div>

<h4>2. Overzetten</h4>

<div class="vc_row">
  <div class="vc_col-sm-3"><label for="aantal-filmspoelen">Aantal filmspoelen</label></div>
  <div class="vc_col-sm-3">[number* aantal-filmspoelen min:0 max:1000 placeholder "0"]</div>
</div>

<div class="vc_row">
  <div class="vc_col-sm-3"><label for="aantal-videobanden">Aantal videobanden</label></div>
  <div class="vc_col-sm-3">[number* aantal-videobanden min:0 max:1000 placeholder "0"]</div>
</div>

<div class="vc_row">
  <div class="vc_col-sm-3"><label for="aantal-dias">Aantal dia's</label></div>
  <div class="vc_col-sm-3">[number* aantal-dias min:0 max:10000 placeholder "0"]</div>
</div>

<h4>3. Opmerkingen</h4>
[textarea opmerkingen]

<h4>4. Hoe hebt u 8millimeter.nl leren kennen?</h4>
[select hoe-leren-kennen include_blank "Kennissen / vrienden / familie" "Google" "Internet, overige" "ANWB Kampioen" "Advertentie" "Mailing" "Radio / TV" "Anders"]

<p>[submit "Volgende"]</p>
[multistep "1-3-http://www.8millimeter.nl/zetom/stap2/"]