Panama Canal Zone Veterans Act of 2021 This bill establishes a presumption of service-connection for specified conditions associated with active duty service in or near the Panama Canal Zone (zone) from January 1, 1958, through the latter of December 31, 1999, or the date on which the last service member departed from duty in the zone. Under a presumption of service-connection, specific conditions diagnosed in certain veterans are presumed to have been caused by the circumstances of their military service. Health care benefits and disability compensation may then be awarded. The presumption of service-connection applies to illnesses listed in the bill (e.g., Hodgkin's disease) or those that (1) have a positive association with exposure to an herbicide agent that is known or presumed to be associated with service in the zone during the specified time period, and (2) become manifest in a veteran who served in the zone during the specified time period and was exposed to an herbicide agent during such service. The Department of Veterans Affairs must prescribe regulations providing for a presumption of service-connection whenever there is a determination that a positive association exists between the exposure to an herbicide agent known or presumed to be associated with service in or near the zone and the occurrence of an illness in humans.