You can view the AWS Backup Jobs console to see if and why the copy jobs failed. This should given you an indication on where to troubleshoot next, as there doesn't seem to be anything wrong with the Terraform code shared.

Some areas that are worth checking, based on the documentation:

• Enable cross-account backup within your AWS Organization and ensure both accounts are members of this organization.
• Review the AWS KMS key policies to ensure your Backup job roles have the right permissions and that they key is accessible from both accounts.
• Review both source and destination vault policies, if any, to see they prevent copying from or to each vault.