Here: https://nypost.com/2021/01/31/biden-administration-unaware-of-20m-covid-19-vaccine-doses-location/

"About 10 percent of that difference, or 2 million doses, is missing because of lags in reporting from states, Biden’s team believes — meaning the remainder could be sitting in warehouses, in freezers, or in transit in the complex distribution network between the federal government and the states."