Can not export any legacy formats. I have enabled photoshop 2021 in the systems security permissions for full disk access and the same for files and folders and accessibility, after having read some of the previous advice on here for older versions of photoshop and OS, but to no avail. An error message appears every time I try to save for web legacy - It's an animated GIF I'm trying to export. An incredibly frustrating fault that needs to be addressed soon!
I ran into this issue and found a solution using information from another forum. For me, it had something to do with Dropbox backups. I turned off the backup of my files that was going to Dropbox and re-tried exporting the photo in legacy format (save for web). This time instead of the error message I got a popup to allow permissions for it to write to the destination folder. I granted it permission and the export was successful. After the successful export I was able to re-enable Dropbox backup