JSFiddle - React, Tailwind, and code Playground

HTML

<button id="BtReg">open</button>
<button class="close">close</button>
<div class="modal">modal</div>

CSS

.modal {
  display: none;

  width: 50px;
  height: 50px;
  background: blue;
}

.open {
  display: flex !important;
}

JavaScript

window.onload = function () {
  const BtReg = document.querySelector("#BtReg");
  const close = document.querySelector(".close");
  const modal = document.querySelector(".modal");

  BtReg.addEventListener('click', function (event) {
    modal.classList.add("open");
  });
  close.addEventListener('click', function (event) {
    modal.classList.remove("open");
  });
}