Spectrum Innovation Act of 2021 This bill specifies a process for auctioning a portion of the electromagnetic spectrum between 3100 megahertz (MHz) and 3540 MHz for nonfederal use, shared federal and nonfederal use, or a combination of those uses. Most wireless technologies (e.g., mobile communication) rely on the electromagnetic spectrum to transmit signals. Specifically, the Office of Management and Budget must transfer funding from the Spectrum Reallocation Fund to federal entities for planning related to the reallocation. The National Telecommunications and Information Administration (NTIA) and the Executive Office of the President must oversee the planning. Informed by the planning, the Department of Commerce must identify at least 200 MHz of spectrum for reallocation. In identifying the spectrum, Commerce must consult with the Department of Defense, the Office of National Science and Technology Policy, and the Federal Communications Commission (FCC). The FCC must (1) adopt rules authorizing the use of the identified spectrum in consultation with the NTIA, and (2) auction licenses for the identified spectrum within seven years of the bill's enactment. Additionally, the President must modify or withdraw (subject to certain limits) current federal assignments of the identified spectrum to increase nonfederal use. The FCC must allow for opportunistic uses (i.e., allow devices to opportunistically identify and transmit on unused spectrum without infringing on the rights of the spectrum license holder) of a withdrawn or modified assignment. An allowed opportunistic use must cease after the auction if the use is inconsistent with the rights of the licensee that obtains its license through the auction.