Ah, I Have a typo in the code.
Use this new code
/* Site background image */
body[class*="collection-type-blog"] {
.section-background, .page-section {
background: transparent !important;
}
& {
background-image: url(https://cdn.pixabay.com/photo/2021/01/07/22/15/dandelion-5898546__340.jpg);
background-size: cover;
background-repeat: no-repeat;
background-position: center center;
background-color: transparent !important;
}}
