Make the Migrant Protection Protocols Mandatory Act of 2021 This bill requires the Department of Justice (DOJ) to remove certain aliens from the United States while such an alien's application for admission is pending. Specifically, if an alien arrived by land from a foreign country bordering the United States and the alien is not clearly and beyond a doubt entitled to admission into the United States, DOJ must return that alien to that bordering foreign country while the alien's application for admission is pending. (Currently, DOJ may choose to detain such an alien or return the alien to the bordering foreign country while the application for admission is pending.)