Extinction Prevention Act of 2021 This bill requires the Department of the Interior to provide financial assistance for the conservation of butterflies and moths in North America; plant species in Hawaii, Guam, American Samoa, and the Northern Mariana Islands; freshwater mussels in the United States; and bony fish found in desert ecosystems in Arizona, California, Colorado, Nevada, New Mexico, and Utah. Among other things, the bill establishes the North America Butterfly Conservation Fund, the Pacific Islands Plant Conservation Fund, the Freshwater Mussels Conservation Fund, and the Southwest Desert Fish Conservation Fund. In selecting projects for assistance, Interior must prioritize projects that conserve threatened and endangered species.