JSFiddle - React, Tailwind, and code Playground

HTML

<body>
<input id="input1" type="text">
<br>
<br>
<input id="input2" type="text">
<br>
<br>
<button>Сложить</button>
</body>

JavaScript

var inputAll = document.querySelectorAll('input');
	var button =  document.querySelector('button');

	function insertAfter(newNode, referenceNode) {
    	referenceNode.parentNode.insertBefore(newNode, referenceNode.nextSibling);
	}

	var p = document.createElement('p');
	p.style.color = 'red';
	

	function errorCheck() {
		for(var i = 0; i < inputAll.length; i++) {
			console.log(inputAll[i].value)
			if (isNaN(+inputAll[i].value)) {
				p.innerHTML = 'error';
				insertAfter(p, inputAll[i]);	
			}
		}
	}

	button.addEventListener('click', function count() {
		p.innerHTML = '';
		var answer = +inputAll[0].value + +inputAll[1].value;
		if (isNaN(answer)) {
			errorCheck();
			return;
		} 
		alert(answer);
	});