1921-S Morgan dollar die adjustment strike, with the coin missing about 25% of it's design on the obverse and reverse, and also very weak reeding on the edge.
The error occurred due to the coin's having been struck with insufficient striking pressure. The press may have been starting up at the time, and the dies might not have been at "full striking pressure" yet. These weakly struck coins are supposed to be thrown away at the Mint, but very rarely one will get through.
There are just a very few known for the Morgan dollar series, with probably less than a dozen examples known for the series.
NGC certified but did not grade the coin because it has a weak strike, and their policy is not to grade coins which are weakly struck. The coin looks to us to be a nice AU.