Can't find S3 bucket using Craft Pro in trial mode
The bucket you're trying to list is probably in the Frankfurt S3 region. The 3rd party library used in Craft 2.x for Amazon S3 support does not support Signature Version 4 that Frankfurt region requires, so you won't be able to use buckets in that region, as outlined in this answer

Craft 3.0 will not have this limitation, however.