The error is caused by a volume type with no partition table and is not supported by Veeam Agent for Microsoft Windows.
In Windows Disk Management, the volume looks like a common MBR single partition. The Virtual Disk service, which Veeam Endpoint Backup uses to detect disks, generates the following messages that are then forwarded to the Veeam log. (log location: C:\ProgramData\Veeam\Backup\Endpoint\Backup_Job_*Machine name*\ Job.Backup_Job_Machine_name.Backup.log).
Info Layout: MBR, disk signature: 1
Info Partition 0, type 04 (FAT16), offset 0
The MBR partition table occupies sector 1 of the disk, so Partition 0 can start no earlier than sector 2. One sector equals 512 bytes for regular disks and 4096 bytes for 4K native hard disk drives. Meaning the offset of Partition 0 must be greater than 512 or 4096.