Failed to insert media "<ISO_NAME>" into VM (VM) "<VM_NAME>". - Cannot insert Media [<ISO_NAME>] into VM [<VM_NAME>] because the VM's host [<HOST_NAME>] doesn't have access to the Media's datastore [<DATASTORE_NAME>].
VMware Cloud Director 10.6.1.1
This is an expected behavior when the catalog is not properly shared between multiple Cloud Director sites.
To help fixing the issue you can either use the Object Storage Extension plugin, see Share a Catalog on documentation.
or You can create a distributed catalogs which are identical on all shared VMware Cloud Director instances because the instances have shared storage. The changes to a distributed catalog can be bidirectional. See Create a Distributed Catalog in the VMware Cloud Director Tenant Portal