Why the burger menu does not pop out?

Question

I tried building a burger-menu that will pop out some items when clicked. I managed to create and style everything but when I click it the menu does not come out. I do not understand why, what suppose to make this happen is clicking on the check-box and it expanding the max-height of the .menu class to a specific height. I am trying to achieve this with pure css.

Here is the relevant code:

The react component I am trying to style:

...
const TopBar = () => {
    ...
    return (
        <header className="header">
            <input className="menu-btn" type="checkbox" id="menu-btn" />
            <label className="menu-icon" htmlFor="menu-btn"><span className="nav-icon"></span></label>
            
            <div className="logo">Sort Visualizer</div>
            
            <section className="menu">
                <div className="algorithm-select">
                    <select name="Sorting Algorithm" 
                            id="algorithm" 
                            onChange={(e) => handleAlgorithmSelect(e)}
                            value={data.sortingAlgorithm}>
                        {sortManager.getAllAlgorithms().map((value) => 
                            <option value={value}
                                    key={shortid.generate()}
                            >{value}</option>)}
                    </select>
                </div>

                <div className="array-size">
                    <div className="slider-container">
                        5
                        <input type="range" 
                               min="5" 
                               max="100"
                               value={data.length}
                               onChange={(e) => handleLength(e)} 
                               className="slider" 
                               id="myRange" 
                        />
                        100
                    </div>
                </div>

                <div>
                    <button className="randomize-btn" onClick={() => generateArray()}>Randomize</button>
                </div>
            </section>
        </header>
    );
}

export default TopBar;

Here is the whole stylesheet:

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  font-size: 62.5%;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Oxygen',
    'Ubuntu', 'Cantarell', 'Fira Sans', 'Droid Sans', 'Helvetica Neue',
    sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

/* TopBar */
.header {
  background-color: #222;
  color: #fff;
  box-shadow: 1px 3px 10px 0 rgba(0, 0, 0, .4);
  position: fixed;
  width: 100%;
  height: 8%;
  z-index: 3;
}

.header .menu {
  clear: both;
  max-height: 0;
  transition: max-height .2s ease-out;
  overflow: hidden;
}

.header .menu div {
  display: block;
  padding: 20px;
  border-right: 1px solid #ccc;
}

.header .logo {
  float: left;
  display: block;
  font-size: 3rem;
  padding: 5px 20px;
}

.header .menu-btn {
  position: absolute;
  top: 0;
  right: 0;
  z-index: 2;
  cursor: pointer;
  width: 45px;
  height: 45px;
  opacity: 0;
}

.header .menu-icon {
  position: absolute;
  top: 0;
  right: 0;
  z-index: 1;
  width: 40px;
  height: 40px;
  margin: 6px 5px;
  display: flex;
  align-items: center;
  justify-content: center;
}

.header .menu-icon .nav-icon {
  position: relative;
  width: 75%;
  height: 2px;
  background: #fff;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: all 0.4s ease;
}

.header .menu-icon .nav-icon:before,
.header .menu-icon .nav-icon:after {
  content: '';
  position: absolute;
  z-index: 1;
  top: -9px;
  width: 100%;
  height: 2px;
  background: inherit;
}

.header .menu-icon .nav-icon:after {
  top: 9px;
}

.header .menu-btn:checked + .menu-icon .nav-icon {
  transform: rotate(135deg);
}

.header .menu-btn:checked + .menu-icon .nav-icon:before,
.header .menu-btn:checked + .menu-icon .nav-icon:after {
  top: 0;
  transform: rotate(90deg);
}

.header .menu-btn:checked ~ .menu {
  max-height: 240px;
}

I tried a few things but it wasn't successful. What am I missing?