The permanently and totally disabled section of the Qualifying Child rules is only to claim a disabled child age 19 or older using the Qualifying Child rules and not the Qualifying Relative rules.
For the Child Tax Credit the child must be under the age of 18.
IRS Schedule 8812 Instructions - https://www.irs.gov/pub/irs-pdf/i1040s8.pdf
Enhanced child tax credit. For 2021, the child tax credit applies to qualifying children who have not attained age 18 by the end of 2021.
