my iMac has a problem turning off

Why not reinstal the MacOS El Capitan instead?

SMC reset,or NVRAM/Pram, will not do anything on your problem?

I am not on Level 6 or 7 yet,but i have some experiences on Mac Computers at last!

Go to the App Store,and get a new MacOs El Capitan for your imac computer!

This will solve your problem,i Think!

try this:

SMC reset      https://support.apple.com/en-us/HT201295

Zap pram   https://support.apple.com/en-us/HT204063

Do you mean it won't turn off or it is slow to turn off? If Smokerz's advice doesn't fixit come back and tell us more.