Alternatives to Detention Act of 2021 This bill requires the Department of Homeland Security (DHS) to establish programs that are alternatives to detention for immigration enforcement and addresses related issues. Such programs must be available regardless of whether an alien is subject to a removal order or whether a charge of removability is pending, but shall not be available to certain aliens, such as those subject to mandatory detention. DHS or an immigration judge must determine, within 72 hours of an individual being taken into custody for immigration enforcement, the appropriate level of supervision for the individual and whether the individual may participate in such an alternative program. For a member of a vulnerable population (such as a victim of trafficking) or a caregiver (such as a parent of a minor), there must be a presumption that such an individual must be placed in a community-based supervision program. DHS must restore the Family Case Management Program as an alternative to detention. This program must provide community supervision and community support services through a contract with a nongovernmental organization. DHS must establish a Coordinator of Alternatives to Detention position. The Government Accountability Office must report on the use and effectiveness of the programs established under this bill.