Check the manufacturer name is correct, the example CSV file downloaded from the portal lists Google as the manufacturer, and resellers may not have changed it.
Confirm the IMEIs are correct.
Confirm a configuration is present and assigned to the device.
Is the device being setup on Wi-Fi? Try cellular to rule out restricted networks.
Is the device connected to a network at all? Check and connect if required.
Is the device running 8.0+? With the exception of Pixel, no device running less than Oreo will support ZT.
Does the OEM support ZT on the device? Unless Android Enterprise Recommended, zero-touch is optional and may therefore not be supported even on 8.0+ devices.