The NVRAM file contains information such as BIOS settings. It is not copied to the target host when a virtual machine is migrated, migrated with VMotion, cloned or deployed from a template.
Changing the boot sequence of the original virtual machine, for instance, modifies the NVRAM settings. Since the NVRAM file is not copied as part of the migration or cloning process, the destination virtual machine might receive a different boot sequence and consequently cannot find the disk with the boot partition.
If you have settings in NVRAM that are not default values, you need to manually copy the original NVRAM file to the target host.