JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <title>TVA belge</title>
</head>
<body>
<form id="f">
    <label for="tva">numéro de TVA: </label>
    <input id="tva" name="tva" type="text" pattern="[01][0-9]{9}" />
    <span id="TVAMessage">erreur</span>
    <br /><br />
    
    <input type="submit" value="vérifier" />
    <input type="reset" />
</form>
</body>
</html>

CSS

* { font-family: monospace; }
#TVAError { display: inline-block; }

JavaScript

const TVAForm = document.getElementById('f'),
      TVAMessage = document.getElementById('TVAMessage');

TVAForm.addEventListener('submit', function(e) {
    const ch = e.target.tva.value,
          pat = /^([01]\d{7})(\d{2})$/,
          m = ch.match(pat);
          
    TVAMessage.textContent =
        m instanceof Array  &&  97 - parseInt(m[1]) % 97 === parseInt(m[2])
        ? 'OK' : 'ERROR';
       
    e.preventDefault(); 
});