Trying to use manual noise reduction but the sliders are greyed out. Any idea why?

As @AxelMatt said. The luminance and color sliders set the amount of luminance and color noise reduction. With the other sliders you can fine tune the effect, but you need to set an amount first, of course.

As far as I see the Luminace andthe Color slider are set to 0. In this case the correspondending sliders are greyed out. Set the appropriate sliders (Lunimance and/or Color) to a value > 0 and the other sliders should be enabled.

If this doesn't help, try to reset the preferences of Lightroom Classic:  How to reset Lightroom Classic preferences (adobe.com)

It's recommended to backup your preferences before you reset the preferences to the default settings: 

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

If this also doesn't help, give us more and detailed informations about the issue and your environment.

Please provide a copy of your 'System Info'. This can be get from the LrC Help > System Info menu item. There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.