plugin does not seem to work

Hi,
I’ve installed the custom background for post and page plugin and I’ve tried to add a custom image on a few of the pages, but it does not work. Is the plugin still compatible with this version of WP, or am I doing something wrong? I defined a background in custom css:
.site
{
background-image: url (…);
}
and that works fine.
If I delete this css, the plugin still doesn’t work. I’ve tried different things, but unfortunately without any results.

Thanks in advance

https://www.ads-software.com/plugins/custom-background-for-post-and-page/