I couldn't create it because "Unauthorized software is installed. Secure Folder can't be used on this phone."But my machine still prompts this in a brand new state. I also checked that the machine Knox is in an unbroken state.
Does Samsung Pass or Health presents you with the same or similar error?
If this is a sudden issue, have you changed Sim cards or installed any software between now and your last successful usage of the software? If so see if swapping conditions back reverses the issue. Not a solution per se but might help you backup stuff that's in secure folder in you need a factory reset.
As for potentially fixes. In settings-> app settings -> secure folder -> storage -> clear cache. In addition to that you can restart your phone in safe mode (long press power off option in shutdown menu)
If it works in safe mode but not in regular mode after a reboot, its pretty fair to say something 3rd party is setting the secure folder off. Which if that feature matters to you leaves with the options of deducing what app is causing the issue, factory resetting, or failing that use PC recovery tools to restore a known system image (this is involved, advanced, and tbh im not sure if it works any more ; it doesn't trip Knox implicitly, but its possible to accidentally do)
