Recognizing and Ensuring Taxpayer Access to Infrastructure Necessary for GPS and Satellite Communications Act of 2021 or the RETAIN GPS and Satellite Communications Act of 2021 This bill places conditions on entities that deploy commercial terrestrial operations in the 1525-1559 megahertz (MHz) band or the 1626.5-1660.6 MHz band of the electromagnetic spectrum. These frequency bands are traditionally used for satellite operations, including the global positioning system (GPS). On April 19, 2020, the Federal Communications Commission approved an application for a low-power 5G wireless network in such bands despite concerns about the network interfering with GPS and other operations on adjacent bands. In its application, the entity committed to update, repair, or replace federal devices, platforms, or systems affected by its operations. This bill requires an entity (including the previously approved entity) that deploys terrestrial operations in the designated bands to reimburse a federal agency or a person for certain costs if the entity's operations interfere with or otherwise affect the operations of the agency or person. These costs may include necessary upgrades, repairs, or replacements to satellite communication devices and related equipment, as well as expenses for engineering studies and labor. Reimbursable costs owed by the entity transfer to any assignee, successor, or purchaser of its license to deploy terrestrial operations in the designated bands. The bill also expressly authorizes federal agencies to directly accept from the entity reimbursements for incurred costs.