Are you doing this as a System Admin? The message you have might be generic because you aren't a system admin. If this was an ISO file uploaded, there is a corner case issue with vCloud 5.5.0 where the ISO extension is lost or placed in a directory too long for ESXi to use. I think both have been corrected in newer versions.
If you're on 5.1.x generation, I'm not aware of any problems.
otherwise, we need to see the full error message. This would be found as a System Admin performing the same action, or via the vcloud-container-debug.log file from the Cell running the vCD website (you might have to check more than one if you have a clustered deployment).