Please open the macOS System Preferences, click Accessibility, and then under the "Seeing" tab, click the button that says Open VoicOver Utility

Next, in VoiceOver Utility, click Speech in the left sidebar, make sure the Voices tab is selected, click the Voice popup menu, scroll to the bottom and select Customize...

Finally, locate the voice that is not appearing in VoiceOver and make sure its checkbox is selected. When you are finished, click the OK button. Now the voice should be available in VoiceOver.