If you want to use Castopod, you almost certainly want to connect it to an S3 bucket. But the documentation on the site is missing a couple critical elements. The bucket must be specified and the format of the host needs the header. Here is an example configuration from the end of the .env file.
media.fileManager="s3"
media.s3.endpoint="https://s3.us-east-001.backblazeb2.com"
media.s3.bucket="bucketname"
media.s3.key="mykeyID"
media.s3.secret="mykeySecret"
media.s3.region="us-east-001"
This took some tracking down. The media.s3.bucket is absolutely critical, but does not appear in the documentation. The https:// I'm told is required as it prefixes bad data without it.