When was your last software update? Was your phone bought locally or
from another country? It could be due to a mismatch region of phone.
Have you tried using Smart Switch on a computer to update the device?
You could also try visiting a Samsung Stor...
If it's not pre-installed, try to search and download Secure Folder from
Google Play Store or Galaxy Store. If you can't still find it or unable
to install it, your device model may not be compatible or does not
support this feature.