Keeping the Lights On Act of 2021 This bill allows an eligible employer a payroll tax credit equal to 50% of a limited amount of qualified fixed expenses paid by such employer in a calendar quarter. Qualified fixed expenses include a mortgage, rent, or a utility payment. An employer is eligible for such payroll credit if the employer was carrying on a trade or business that had not more than 1,500 full-time employees or not more than $41.5 million in gross receipts in 2019. The employer's business must have been suspended in a calendar quarter due to COVID-19 (i.e., coronavirus disease 2019) and have experienced a significant decline in gross receipts due to that pandemic. The credit may not be claimed by the federal government or by any state or tribal government, or by U.S. possessions, but may be claimed by tax-exempt organizations.