update your .left class like this
.left {
order:2;
background-color:#3c3c3c;
overflow-y: auto;
width:auto;
z-index:1000;
position:relative;
}
and .bg like this
.bg { background: url(https://cdn.wallpapersafari.com/30/25/1fhUk9.jpg);
position: fixed;
background-attachment: fixed;
background-size: cover;
background-color: #00000033;
background-position: center;
inset: 0;
z-index:-1;
position:absolute;
}