Sometimes when creating an S3 bucket, it appears that sometimes the buckets are not available right away. This can go through and cause issues with validating backup settings for the bucket.
So far, based on previous reports, the S3 backups can take up to 24 hours to fully become available for backups. ›We recommend waiting 24 hours before trying to validate the S3 bucket again. You can validate it through WHM in Home >> Backup Configuration >> Additional Destinations.
If you would like to use WHM's API, you can also use the following command:
whmapi1 backup_destination_validate id=DESTINATION-ID disableonfail=0
You will need to replace the DESTINATION-ID with the correct ID located in Backup Configuration's Additional Destinations for it to work. From there, you can see through both ways if the bucket is ready for use or not.