JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper">
    <div class="sidebar">
        <div class="main_content">
            <div class="header">
                <a href="#">Sair</a>
            </div>
            <div id="conteudo">
                <form class="form" method="post" action="#">
                    <div class="area">
                        <fieldset>
                            <legend>Cadastrar Computadores</legend>

                            <div class="campo">
                                <label for="nome">Nome</label>
                                    <input type="text" name="nome" id="nome" placeholder="Digite o nome" required>
                            </div>

                            <div class="campo">
                                <label for="status">Status</label>
                                <select name="status" id="status" required>
                                    <option value=""></option>
                                    <option value="#">Habilitado</option>
                                    <option value="#">Desabilitado</option>
                                </select>
                            </div>

                            <div class="campo">
                                <label for="comentarios">Comentários</label>
                                <textarea rows="8" cols="60" maxlength="500"></textarea>
                            </div>

                        </fieldset>

                        <input type="submit" name="Enviar" value="Enviar">
                        <input type="reset" name="Limpar" value="Limpar" style="margin-right: 15px;">
                    </div>
                </form>
            </div> 
        </div>
    </div>
</div>

CSS

/* Menu */
@charset "UTF-8";
*{
margin: 0;
padding: 0;
box-sizing: border-box;
list-style: none;
text-decoration: none;
font-family: 'Arial', sans-serif;
}

.wrapper{
display: flex;
position: relative;
}

.wrapper .sidebar{
height: 100%;
background-color: #1D2731;
position: fixed;
border-right: 1px solid #0B3C5D;
overflow: auto;
}

.wrapper .main_content{
margin-left: 250px;
top: 0;
}

.wrapper .main_content .header{
padding: 21px;
background-color: #0B3C5D;
position: fixed;
width: 100%;
z-index: 1;
}
.wrapper .main_content .header a{
color: #fff;
}

/* Conteúdo Corpo*/
*{
padding: 0;
margin: 0;
}
.form {
display: flex;
width: 50%;
/* margin-left: 350px; */
}

.campo {
width: 40%;
float: left;
margin: 10px;
}

fieldset {
padding-left: 50px;
}

.campo input {
margin: 10px 1%;
width: 60%;
}

.campo select {
margin: 10px 1%;
width: 60%;
}

label {
display: inline-block;
width: 70px;
font-size: 14px;
}

label:after {
    content: ':';
}

legend {
text-align: center;
font-weight: bold;
font-size: 13pt;
}

input[type=text], input[type=number], input[type=date],
input[type=email], input[type=password], select {
padding: 5px;
border: 1px solid #ccc;
border-radius: 4px;
}

input:hover {
background-color: #D3D3D3;
}

input[type=submit] {
background-color: #4CAF50;
color: white;
padding: 12px 20px;
border: none;
border-radius: 4px;
cursor: pointer;
float: right;
margin-right: 15px;
margin-top: 10px;
}

input[type=submit]:hover {
background-color: #008000;
}

input[type=reset] {
background-color: #FF6347;
color: white;
padding: 12px 20px;
border: none;
border-radius: 4px;
cursor: pointer;
float: right;
margin-top: 10px;
}

input[type=reset]:hover {
background-color: #FF0000;
}

input[type=checkbox] {
margin: -60px;
}

span {
padding: -5px;
border:none;
}

#conteudo{
    background: #fff;
    position: relative;
    top: 60px;
    display: flex;
    justify-content: center;
    height: 100%;
    padding-top: 30px
}

JavaScript

const conteudo = document.getElementById('conteudo');
const header = document.querySelector('.header');
conteudo.style.width = screen.width - 250;
header.style.width = screen.width - 250;