Reporting Efficiently to Proper Officials in Response to Terrorism Act of 2021 or the REPORT Act This bill requires specified federal agencies to report to Congress concerning any act of terrorism that occurs in the United States. Whenever an act of terrorism occurs, the Department of Homeland Security (DHS), the Department of Justice (DOJ), the Federal Bureau of Investigation (FBI), and, as appropriate, the National Counterterrorism Center (NCC), must submit to the appropriate congressional committees, as specified by this bill, an unclassified report not later than one year after the completion of the investigation by the primary agency conducting the investigation concerning the terrorist act. Such report shall (1) include a statement of the facts of the act of terrorism that are known at the time of such report; (2) identify any gaps in homeland or national security that could be addressed to prevent future acts of terrorism; and (3) include any recommendations for additional measures that could be taken to improve homeland or national security, such as changes in law enforcement practices or changes in law. If DHS, DOJ, and the FBI or, as appropriate, the NCC, determines any information required to be reported could jeopardize an ongoing investigation or prosecution, such entities (1) may withhold from reporting such information, and (2) shall notify the appropriate congressional committees of that determination.