Build error

Error message:
--2018-06-02 13:50:42-- http://archive.ubuntu.com/ubuntu/ubuntu/ubuntu/pool/main/g/gfxboot-theme-ubuntu/gfxboot-theme-ubuntu_0.20.1.tar.xz
Resolving archive.ubuntu.com (archive.ubuntu.com)... 91.189.88.149, 91.189.88.161, 91.189.88.152, ...
Connecting to archive.ubuntu.com (archive.ubuntu.com)|91.189.88.149|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 196616 (192K) [application/x-xz]
Saving to: ‘gfxboot-theme-ubuntu_0.20.1.tar.xz’

     0K .......... .......... .......... .......... .......... 26% 165K 1s
    50K .......... .......... .......... .......... .......... 52% 349K 0s
   100K .......... .......... .......... .......... .......... 78% 619K 0s
   150K .......... .......... .......... .......... .. 100% 709K=0.6s

2018-06-02 13:50:43 (328 KB/s) - ‘gfxboot-theme-ubuntu_0.20.1.tar.xz’ saved [196616/196616]

gzip: stdin: not in gzip format
tar: Child returned status 1
tar: Error is not recoverable: exiting now
Unable to extract gfxboot-theme-ubuntu source package

I have already made these changes;

Ubuntu Customization Kit has not been developed for years, and has the note
"!!!PROJECT DISCONTINUED!!!"
on its Launchpad homepage https://launchpad.net/uck

Recent Ubuntu releases differ from previous ones in a manner, that certain steps of UCK now fail.

I can give some advice:
     1. deselect all language packages that contain the string -touch- in their name
     2. modify /usr/lib/uck/customization-profiles/localized_cd/customize_iso and change in line ~84 (the longest line in the whole file) the part

... $(zgrep gz ...
      into
... $(zgrep xz ...

and three lines later
      change

tar xfz *.tar.gz ||
       into
tar xfJ *.tar.xz ||

You might read recent questions and bugs related to UCK on Launchpad. These recommendations (and some more) have already been given to other persons with similar problems.