Hemp Economic Mobilization Plan Act of 2021 or the HEMP Act of 2021 This bill modifies the definition of hemp to raise the tetrahydrocannabinol (THC, the psychoactive component of marijuana) limit from 0.3% to 1%. Additionally, the bill requires the Department of Agriculture (USDA) and state and tribal governments to test hemp-derived products rather than the hemp flower or plant itself and creates a margin of error in hemp testing using 0.075% as the baseline standard. Further, it requires hemp shipments to contain (1) a copy of a valid license or other authorization from USDA or a state or tribal government, or (2) a copy of a certification from a laboratory showing that such hemp contains a THC concentration of not more than 1% on a dry weight basis.