There seem to be two issues here. (If you only encountered one of this issues, you would not have experienced this particular bug).
ISSUE #1: ---------
Cubic cannot identify the version of the initrd (named "initrd") file on the original ISO you are trying to customize.
ISSUE #2: ---------
Cubic does not find a vmlinuz or initrd file in your customized file system.
There seem to be two issues here. (If you only encountered one of this issues, you would not have experienced this particular bug).
ISSUE #1:
---------
Cubic cannot identify the version of the initrd (named "initrd") file on the original ISO you are trying to customize.
ISSUE #2:
---------
Cubic does not find a vmlinuz or initrd file in your customized file system.