can't unlock my iMac with my Apple Watch

Have you tried toggling auto-unlock option off, restart your iMac, toggle the auto-unlock on, restart and test? You might also try restarting your iMac in Safe Mode (hold down the Shift key when restarting, this could take up to 10 minutes) and test?

If you can’t or choose not to try troubleshooting then contact Apple Support with the link at the top right of this screen.

More details please. What version of macOS are you running? What version of watchOS are you running and what model of Watch are you using? There have been issues like this in the past but Apple released fixes some time ago.

Make sure your iMac and Watch are updated to the latest software.

Also use the link at the top of this page and follow the instructions to find out if your iMac supports out-unlock.