The SEE Client Administrator and SEE Management Agent will both show no internal drives if Disk 0 is unallocated (unformated) in a system with two or more drives. I have an HP ZBook 15 that came with an internal 256 GB M.2 PCI-E SSD. That drive proved to be too small for my needs so I installed an additional 1 TB SATA SSD. I left the M.2 drive in the laptop when I added the new SSD. Disk Management shows the 256 GB SSD as Disk 0 (Unallocated) and the 1 TB SSD as Disk 1. Windows 10 1809 is installed on Disk 1. If I create a new simple volume on Disk 0 and then launch either the SEE Client Administrator or SEE Management Agent I will see both Disk 0 and DIsk 1 displayed. If I delete the volume on Disk 0, the SEE Client Administrator and SEE Management Agent will both fail to show any internal drives. This was tested with both Symantec Endpoint Encryption 11.2.1 and 11.2.1 MP1. I was able to verify that Disk 1 is in fact encrypted with the following command.
C:\Program Files\Symantec\Endpoint Encryption Clients\Drive Encryption>eedadmincli.exe --status