Pretty sure its either an AT&T bug in their newest security update because everyone else that has this problem is on AT&T. It's either that or it is a virus, download Malwarebytes and get the free trial, (it wont charge you when it ends), and do a full scan. If there is a virus then delete it on malwarebytes and if it ever comes back then back up your device and do a factory reset. But I would expect for most people that it is an AT&T problem.
I see out there that lots of folks across multiple devices, mostly ATT customers, have recently been getting a random message (little grey oval) that reads "Can't install app. Not optimized for secure folder" or something very close to that. What's going on and how do we make it stop. I'm on a fully updated S9 with ATT.
I am also having this issue on my AT&T S9. I force stopped and cleared the cache of most apps, rebooted, nothing I've done fixes it. Started after the latest security update, must be a bug in the patch. I'm really surprised to see no comments from Samsung on this yet.