Won
My iMac 27", OS High Sierra, Ver 10.13.6, 2011
Can't get it to go to sleep, and seems to be causing other issues as well.
iMac 27", macOS 10.13
When having sleep issues:
1) disconnect all USB devices except mouse and keyboard and test. Some older USB devices are not up to standards and can prevent sleep.
2) Try and SMC reset. Please follow the instructions in this Apple article:
How to reset the SMC of your Mac - Apple Support
3) Have you installed any anti-virus, "cleaning,: or torrent software?
4) Boot into Safe Mode and see if the problem goes away, per this Apple article: