JSFiddle - React, Tailwind, and code Playground

HTML

<form type="POST" onsubmit="return absenden()">
    <input type="radio" name="radio-button" id="checkbox1">
    <input type="radio" name="radio-button" id="checkbox2">
    
    <button type="submit">
        Submit
    </button>
</form>

JavaScript

function absenden() {
  var radio = document.querySelector('input[name = "radio-button"]:checked');

  if (radio == null) {
    alert("Bitte einen Button auswählen!");
    return false;
  }
}