I cannot get my AirPods to connect to my iMac no matter what I try

What year iMac is it, and what macOS version is it running?

You can find that info by clicking the  (Apple) menu at the top of your screen and then select About This Mac.

Start by resetting the AirPods and see if you can re-pair them with the iMac.

How to reset your AirPods and AirPods Pro - Apple Support

Also, please check these resources for additional info:

Set up AirPods with your Mac… - Apple Support

Connect a Bluetooth device with your Mac - Apple Support