iMac keeps shutting down from sleep mode

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Disable Sleep, it’s unnecessary on later Macs, all pain, no gain. To disable, set your Energy Saver settings to as shown: 

Separately, avoid manual sleep, use screen lock instead or let the display switch off/auto-lock on timer. For long or overnight absences, shut.