Apple Cash Family - "Family member already has Apple Cash enabled"
This can happen if the minor has previously created an Apple Cash account but was unable to verify it due to not meeting the age requirements. The account does not show up because it is not active, it cannot be enabled because they do not meet the age requirement, and yet it prevents them from having an Apple Cash Family card created on their behalf.
To resolve this issue, you can call Apple Pay support at (877) 233-8552. Once they verify the minor's identity, they will be able to close the empty account so that you can create a new one.