Sometimes security updates fail, but the first thing to check is to be sure you are in fact on 10.14.6 and not some earlier revision of Mojave. You might also try a "safe boot" that disables third-party startups and such and see if you can install it that way. Also, check to make sure you have LOTS of free space -- that's the #1 reason updates fail.
I have a 2012 MBP that I'm keeping on Mojave for some old games and one particular no-longer-supported audio plugin I need for my podcast. Since it is still supported with security updates, I'm able to keep it online ... but at some point pretty soon it will no longer get those updates and I will be using it offline-only (or giving up those unsupported apps and upgrading to Big Sur).
Of course I have the luxury of a more recent MBP running Big Sur for my "real" work. Sure makes the 2012 seem a lot slower than it actually is, but it's been a very solid machine for me for a long time.