No. Lr-Classic is not a 'pixel' editor that can replace parts of an image with 'white'.
A white 'mask' is just that- a mask- an overlay- that indicates where you are applying an adjustment (Brush, Gradient).
You can however select the subject, invert the mask (to affect the background), then use the adjdustment sliders to go to extremes of Exposure +4, etc.
Ideally Photoshop is the tool for this task.
Having applied the adjustments (the mask disappears) then you need to Export the image, or [Edit-In] Photoshop to finish off.
... View more
AdChoices