dodge an burn overlays not showing

Holding down the Option key for a black-and-white preview is used when adjusting options such as Exposure, Highlights, etc. (for clipping), or the Amount or Masking options for Sharpening in the Detail panel. 

If you’re dodging and burning with mask tools like the Brush, in the current version (12), the mask overlay is controlled by the settings on the Tools > Mask Overlay submenu. On that submenu you can see that the keyboard shortcut for toggling the Show Overlay command is tapping the O key. If you hold down the O key instead, it toggles on as long as the key is held down, and toggles off when you release the key, the same way the Before and After and other toggle keys work in Lightroom Classic. Or, hover the pointer over the mask thumbnail.

You can also set the overlay type on that submenu, like if you want it to show in black and white only. Some of those options are also on the Mask panel itself; the Show Overlay checkbox is the same as the Tools > Mask Overlay > Show Overlay command and uses the same O shortcut.

If the toolbar is displayed at the bottom of the window, you can see that the Overlay Mode menu appears there too.

If you are using a Brush in the Masking panel, then the [Alt] key swaps the Brush to 'Erase' mode.

If you are using the Healing Brush Tool  (aka Spot Removal tool) then keyboard [A] will activate the Visualize B&W mode, as will the Visualize checkbox on the lower Tool bar.

Otherwise explain in more detail your workflow and meaning for "dodge and burn".