Hello,
I am not a Linux user, so I cannot do test, but for what I know:
1) DJ Control Steel was not a USB MIDI compliant device, so it requires installing drivers
2) I have just uploaded the Linux drivers now on https://support.hercules.com/en/product/djcontrolsteel-en/ , in the drivers > View more section
3) These drivers are very old, they were made on 32-bit OS, so I do not know whether today's Ubuntu support these old drivers.
4) As the Hercules ftp server did not recognize the tgz format, the file is in zip, so you have to install the drivers manually.
5) Mixxx used to support DJ Control Steel in Linux, I do not know how it works today in Linux.
You will find more useful informations on the following pages to get your controller working with Mixxx under Linux:
https://www.mixxx.org/wiki/doku.php/hercules_linux_usermode_driver
https://www.mixxx.org/wiki/doku.php/hercules_linux_kernel_module
https://www.mixxx.org/wiki/doku.php/hercules_dj_control_steel
