Sorry the program was correct ... for 2020 the child had to be UNDER the age of 17 ... for the 2021 return they had to be UNDER the age of 18. So you lost both years. All you get is the other dependent $500 deduction.
I know it's frustrating but hopefully the link below to an article will provide some clarity on the credit for 2020 & 2021. Also, the article does provide some info on the Other Dependent Credit. Hope this helps.