The message you get means that you gave a boot option mentioning
isolinux.bin as the boot image, but that the path to isolinux.bin is not
in the list of files which shall show up in the emerging ISO image.
-boot_image is the xorriso command which controls many boot related
properties. If you did not use it then I guess you used the mkisofs
emulation, where this was probably option -b (alias -eltorito-boot) which
expects the path inside the ISO, not in the filesystem from where your
files come.
So make sure to give with -b the path of isolinux.bin in the ISO.
In many ISOs, you will find it in /boot/isolinux or in /isolinux.
So if you just pack up a mounted ISO again, you will possibly have success
with
-b boot/isolinux/isolinux.bin
or
-b isolinux/isolinux.bin
(The lack of trailing slash makes these paths compatible with mkisofs which
expects a relative path in the working directory of the source filesystem.
I.e. with mkisofs -b would work only if your pwd is the ISO's root.)
I could give clearer advise if you tell me the URL of the ISO which you
try to use as input and the xorrisofs options which you used.