State Border Security Reimbursement Act of 2021 This bill requires the federal government to reimburse eligible states for their border security expenses. To be eligible, a state must have expended more than $2.5 billion on border security and enforcement in the 10 years before this bill's enactment. If such a state provides an accounting of all of its nonfederally funded border security expenses any time up to 180 days after this bill's enactment, the federal government shall reimburse the full amount.