JSFiddle - React, Tailwind, and code Playground

HTML

<label style="color: #993300"> Ваш логин:</label>
 
<span style="font-size: medium;"><input maxlength="15" name="login" id = "login" class="sf" size="15" type="text" /> <span id="message"></span></span>

JavaScript

var input = document.getElementById('login');

	input.oninput = function() {
    
    var message;
    
    if (input.value.length < 5) {
       message = 'Логин слишком короткий';
    } else {
       message = '';
    }  
	if (input.value.length == 0) {
       message = '';
    }	
    document.getElementById('message').innerHTML = message;
    
    
}