Helping to Encourage Real Opportunities (HERO) for Youth Act of 2021 This bill modifies the work opportunity tax credit to (1) change the credit for summer youth employees to a credit for youth employees who will be employed for not more than 20 hours per week during any period between September 16 and April 30 in which the youth employee is attending any secondary school, (2) increase the amount of the credit for youth employees, and (3) expand the credit to include disconnected youth. The bill defines disconnected youth to include any individual who (1) is certified as having attained age 16 but not age 25 on the hiring date; and (2) has self-certified as not having regularly attended any secondary, technical, or post-secondary school during the 6-month period preceding the hiring date, has not been regularly employed during such period, and is not readily employable due to a lack of basic skills. The term also includes individuals who have been certified (1) as having attained age 16 but not age 21 on the hiring date, and (2) as eligible foster children in foster care during the 12-month period ending on the hiring date.