Ive had this mini for about 8 months now It’s running Sequoia 15.8.1
I wanted to add a folder to the side bar, something I’ve done on all the Macs I’ve ever owned. (I’m writing this on a Tahoe MacBook and it’s fine on here)
Are you using the drag-and-drop method or the Command-Control-T method for adding the folder to the Favorites section of the Sidebar? Is there anything you have selected in Finder>Preferences>Sidebar in Tahoe that is not selected in Sequoia?