Promoting Readiness and Ensuring Proper Active pharmaceutical ingredient Reserves of Essential medicines Act of 2021 or the PREPARE Act This bill establishes the Strategic Active Pharmaceutical Ingredient Reserve. The Department of Health and Human Services (HHS) must annually publish a public list of essential generic medicines that are medically necessary to have available at all times. HHS must report and implement a plan to establish the reserve, which must be designed to minimize the impact of an interruption or reduction in imports of (1) key starting materials (including active pharmaceutical ingredients) for essential generic medicines, and (2) finished dosage forms of essential generic medicines for which key starting materials are not imported. The plan must also contain provisions to strengthen domestic capacity for active pharmaceutical ingredient production, storage, and conversion. Within one year of this bill's enactment, the plan must include at least 25 of the essential generic medicines on the required list. The plan must add at least 25 medicines each year until the full list is covered. HHS must prioritize domestically manufactured key starting materials for the reserve to the maximum extent possible. A manufacturer of an essential generic medicine that switches to an active pharmaceutical ingredient sourced through the reserve shall not have to update the approved application for that medicine. The Government Accountability Office must report to Congress on active pharmaceutical ingredient manufacturing, including the time and costs needed to develop domestic manufacturing capabilities.