Computer won't sleep

My Mac mini will not go to sleep, even after I select "Sleep" in the Apple menu. This problem began immediately after I installed Security Update 2018-001 to Mac OS 10.12.6. What can I do to stop it from wearing itself out, other than shutting it down after each use?

Specs: Mac mini, Late 2014 version, with 1.4 GHz Intel Core i5 processor and 4 GB 1600 MHz DDR3 memory.

The only peripheral devices connected are a standard keyboard, wired mouse, and monitor.