If you are (suddenly) unable to save on Mac, please check that Fade In has access under System Settings > Privacy & Security > Files & Folders.

(It may be necessary to add Fade In to Full Disk Access before you can add it to Files & Folders, even if Full Disk Access is not enabled for Fade In.)

Recent versions of macOS have begun asking (or asking again) for a particular application, like Fade In, to access files for opening and saving. It is possible to inadvertently disable this by dismissing the popup when the system asks for permission.