JSFiddle - React, Tailwind, and code Playground

HTML

<body>
<form>
    <div id="letter"></div>
    <input type="text" value="" id="city" >
    <button id="play">Играть!</button>
</form>

JavaScript

window.onload = function() {
	var ab = ["а", "б", "в", "г", "д", "е", "ж", "з", "и", "й", "к", 	"л", "м", "н", "о", "п", "р", "с", "т"];

    var LettersDiv = document.getElementById('letter');
    var playBtn = document.getElementById('play');
    
        playBtn.addEventListener('click',function(){
        var rand = 0;
        rand = 0 + Math.floor(Math.random() * (ab.length + 1));
        LettersDiv.innerHTML = ab[rand];

    })
    }