An essential feature of physicalism is that it explains appearances in terms of their underlying physical reality.
For example, the appearance of lightning can be reduced to the reality of electrical discharge.
But it doesn't make sense to ask what conscious experience reduces to, as the distinction between appearance and reality does not apply to consciousness.
Within consciousness the appearance is the reality. So consciousness can't be reduced in the normal scientific manner as "nothing" but some other physical process.
Thomas Nagel (1974).