That appears to be a quite common problem. We have this thread here where folk are discussing it.
Annoying Message "Prevention Information"
5 pages worth.
As far as I have read of it, it appears to be in N3's everywhere and the idea seems to be that it is something Samsung is going to have to address that is related to Knox.
I checked that 30 day thing, have disabled all of the Knox stuff and got an app called App ops https://play.google.com/store/apps/details?id=com.schurich.android.tools.appopsstarte r&hl=en
and have shut off various things connected to Knox, as well. Still in that 30 days though, so I don't know if I have fixed anything or the message is just being inhibited.
Glad you like it here, and welcome! Glad you got TWRP installed!