I can't get dictation to work on my iMac
Have you booted in Safe mode and then rebooted normally?  Have you used Disk Utility to Repair Disk?  These are common quick fixes for a lot of nuisance problems.

Have you checked Sound preferences to make sure your mic is enabled?

Do you have the same problem in your bootable backup clone?

Has Dictation ever worked?  If so, what changed immediately before it stopped working?

Have you tried booting into Recovery mode and reinstalling the OS?