Workflex in the 21st Century Act This bill amends the Employee Retirement Income Security Act of 1974 (ERISA) to establish a voluntary workflex option under which employers who provide flexible workplace arrangement plans that include a combination of paid leave and flexible work options are exempt from certain state and local laws regarding employee benefits. A flexible workplace arrangement plan must provide all employees with a minimum amount of paid leave per year that ranges from 12-20 days, depending on the size of the employer and the tenure of the employee. The plan must also provide employees that meet certain service requirements with at least one of the following flexible work options: a biweekly work program, a compressed work schedule, a telework program, a job sharing program, flexible scheduling, or predictable scheduling. The bill also specifies various requirements for paid leave and the flexible work options.