iMac will not sleep

Here’s what Apple has to say about Sleep issues:

If your Mac doesn't sleep or wake when expected - Apple Support

FWIW, I gave up on the Sleep function years ago. Ever since the inception of OS X there have been Sleep related issues. And even more lately. I just use Display Sleep and keep it set for 15 minutes so I'm not burning the screen. That way your Mac uses very little power over Sleep and is ready to use at the touch of a key without any grogginess. UNIX based systems were made to be always on and they’ve been running busy servers 24/7 for decades.

Reset the NVRAM and if necessary restart in Safe Mode then restart normally and re-test.

How to reset NVRAM on your Mac

Use safe mode to isolate issues with your Mac - Apple Support

Try disconnecting any USB devices. I had sleep problems caused by a USB joystick.

You're welcome, I don't think you'll regret it.