AlexeyPonomarev42,
что не так?
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.hidden {
display: none;
}
</style>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
<script>
$(function() {
const modal = $('#modal');
const modalBtn = $('#btn_own');
const modalBtnClose = $('#modal_btn_close');
modalBtn.click(function() {
modal.removeClass("hidden");
});
modalBtnClose.click(function() {
modal.addClass("hidden");
});
});
</script>
</head>
<body>
<button id="btn_own">open</button>
<div id="modal" class="hidden">modal<button id="modal_btn_close">close</button></div>
</body>
</html>