Unfortunately Apple seems to be actively working to make sure that the only way an iPod can be loaded with music is by using iTunes which is only supported on Mac or Windows.
You have a few of options on how to move forward:
1: Make a Windows virtual machine, install iTunes onto it and pass the iPod though to the VM.
2: Install Rockbox (if able) onto the device to enable it to act as a USB mass storage device allowing drag n drop loading of music.
3: Sell the iPod and get one of the many different digital audio players available on the market as most are OS agnostic (they show up as a USB mass storage device) and most use MicroSD cards to store the music meaning you can move the card to a new player as you upgrade later (so you are not locked to one vendor).