If an NVR shows the error message, shown below, on the screen when powered on and doesn't boot, it can be due to a faulty HDD connected to the NVR
MODSIGN: Couldn't get UEFI db list
The screenshot was taken from a XRN-820S that had a faulty HDD connected to SATA3
Disconnect any HDDs from the NVR and try booting without any HDDs connected
If the NVR boots, try connecting one HDD at a time until the faulty unit is found
Replace any faulty HDDs with new compatible HDDs