It depends on the version of the Snap Server. The older SNAPOS servers you can not install backup agents onto them. You will always get a BackupExec error saying that BackupExec was unable to use the agent, because there was no agent. This is trivial and does not mean anything to worry about.
If you are using the Guardian OS Snap Servers then you can install the Unix / Linux agent and backup as a Unix Client.
User Defined section will also work.
One very important step to know, if by chance you are using the Guardian OS Snap Server, do not turn on NDMP or change the port that NDMP runs on. The Unix Agent uses NDMP running on port 10000 and you will get a backup failure every time.