Creating a cross-account copy of a recovery point from AWS Backup requires the correct access policies on both source and destination vault, correct IAM role with policies that allows to create a copy job and cross-account backup to be enabled within the Organization's management account.

From the details provided in the comments, you seem to have everything except enabling cross-account backup within the AWS Organization. This can be done from the Management Account, within the AWS Backup console under My account and Settings.

Lastly, a final point to check, the destination vault cannot be the default vault of the account.