Check your knox status first-
Download this app and chek the Knox warranty status.If its 0×1 then you can't use secure folder anymore.
Or if it's 0×0 then it might be problem with last update.Try to do a factory reset or flash official firmware through Odin or do it in service center and then check the isuue.
If need any help then reply back.
0×1 means you have tripped knox due to unofficial things like Rooting your device.So now you can't use Secure folder or any other Knox apps anymore.(Knox is an Efuse in motherboard,once tripped then it can't be revert back to its orignal state and it is necessary for running secure folder)
For more info visit-
Warranty bit info-
Kabhi Odin se firmware flash kiya hai?
kabhi kabhi firmware flash krne se bhi knox trip ho jata hai.(Wrong firmware ya fir wrong command in Odin)