Check that the bucket has the correct AWS policy, which grants Carbon Black Cloud the ability to write objects to the bucket
A sample policy can be found here: https://docs.vmware.com/en/VMware-Carbon-Black-Cloud/services/carbon-black-cloud-user-guide/GUID-96EF04B1-440A-4769-BE1A-2BE3E517E022.html
The most common issues are:
The bucket policy was never applied
The bucket policy does not have the correct Principal ID in the Statement -> Principal -> AWS field. A table of Principal IDs is available in the documentation linked above.
The bucket policy doesn't specify the correct bucket name under "Resource" or is missing a wildcard after the bucket name or prefix
Check that the bucket isn't using unsupported encryption
Carbon Black Cloud does not currently support KMS encrypted buckets, only AWS managed encryption