big sur – How can I change my desktop background? I’ve tried everything with no luck

Unless you’re trying from a user account with restricted permissions, you should be able to change the background image by going to:

System Preferences > Desktop & Screen Saver > Desktop

There, you will have default choices, including those supplied by the system, or Photos, and some folders. You should be able to drop your image into one of the listed folders, or add your own.