failed o build live cd

tried to make custom ubuntu 18.04.1 liveCD with 'pl' language package, no windows stuff and with CD/USB option on and got this log:

Build (/usr/bin/uck-gui --wait-before-exit) started at 2018-12-11 16:59:03
>> Ubuntu Customization Kit 2.4.7 on Ubuntu 18.04, 4.15.0-42-generic x86_64
Starting CD remastering on wto, 11 gru 2018, 16:59:09 CET
Customization dir=/home/silver/tmp/customization-scripts
Removing remastering root dir...
Removing ISO remastering dir...
Mounting ISO image...
mount: /home/silver/tmp/remaster-iso-mount: WARNING: device write-protected, mounted read-only.
Unpacking ISO image...
Unmounting ISO image...
Mounting SquashFS image...
Unpacking SquashFS image...
Unmounting SquashFS image...
Removing win32 files...
Creating root home...
Mounting /proc
Mounting /sys
Mounting /dev/pts
Mounting /tmp
Mounting /home/silver/tmp/remaster-root-home
Mounting /home/silver/tmp/remaster-apt-cache
Mounting /run
Mounting /home/silver/tmp/customization-scripts
Copying fstab/mtab...
cp: '/etc/mtab' and '/home/silver/tmp/remaster-root/etc/mtab' are the same file
Unmounting /home/silver/tmp/remaster-root/var/cache/apt...
Unmounting /home/silver/tmp/remaster-root/tmp/customization-scripts...
Unmounting /home/silver/tmp/remaster-root/tmp...
Unmounting /home/silver/tmp/remaster-root/sys...
Unmounting /home/silver/tmp/remaster-root/run...
Unmounting /home/silver/tmp/remaster-root/root...
Unmounting /home/silver/tmp/remaster-root/proc...
Unmounting /home/silver/tmp/remaster-root/dev/pts...
Failed to copy mtab, error=1
Gtk-Message: 17:02:13.238: GtkDialog mapped without a transient parent. This is discouraged.

Is this the error what I'm looking for:
mount: /home/silver/tmp/remaster-iso-mount: WARNING: device write-protected, mounted read-only.
??
or maybe this one:
cp: '/etc/mtab' and '/home/silver/tmp/remaster-root/etc/mtab' are the same file
Failed to copy mtab, error=1
??

Or maybe this fail is caused by something else?