JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/air-datepicker/2.2.3/js/datepicker.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/air-datepicker/2.2.3/css/datepicker.min.css">
 <form action="" class="reg__form">
    <div class="reg__box">
     <input type="checkbox" id="reg__years" class="reg__input">
     <label for="reg__years" class="reg__label">мне есть 18 лет.* </label>
     <p class="reg__error reg__error--years" style="display: none;">оберите вариант</p>
   </div>
   
   <div class="reg__box">
     <input type="checkbox" id="reg__agree" class="reg__input">
     <label for="reg__agree" class="reg__label">Я соглашаюсь с уловиями.* </label>
     <p class="reg__error reg__error--agree" style="display: none;">оберите вариант</p>
   </div>
   <button class="btn">btn</button>
 </form>

CSS

option {
  display: block;
  padding: 10px;
}

.reg__error {
  color: red;
}

JavaScript

function checkCheckBox(elem, error) {
  if ($(elem).prop('checked')) {
    $(error).css("display", "none");
  } else {
    $(error).css("display", "block");
  }
}

$('.btn').on('click', function() {
  checkCheckBox('#reg__agree', '.reg__error--agree');
  checkCheckBox('#reg__years', '.reg__error--years');
  return false;
});


$('form').on('change', function() {
  checkCheckBox('#reg__agree', '.reg__error--agree');
  checkCheckBox('#reg__years', '.reg__error--years');
});