Cannot partition my Disk: Free space in disk utility much smaller than actual free space
is it an apfs vol?
did u try to partition the volume without a specified format?
please post more info !!
off the top of my head i can think of a couple of things, mainly 

diskutil eraseVolume /dev/diskXY (where X= the disk and 1 the volume within that disk) "Free Space" "%noformat%" &lt;desired amount, %, or if you've already created a separate volume use the R option or 100% in order to erase the entire volume&gt;

diskutil partitionDisk /dev/disk2 1 GPT "Free Space" "%noformat%" "100%" 
or format it as a FAT32 part and use that for linux(its been a while since i've done this myself; but i can find u some resources if u'd like...)



  
    check the man pages, man diskutil 
    
    also what mac are u using?(make model and what not ... it might be handier to install refit/refind and use that as boot manager before installing ubuntu so that your mac will always boot up into a 
     custom bootloader with a choice of operating systems).
    
    if none of these are viable options or you'd rather not mess with the bootloader, why not just install vmware fusion, virtualbox and use ubuntu as a virtual machine (depending on what you'll be using it for, not too much resources need to be allocated; i'd start off with 2cpu and around 2048-4096mb of ram[this, of course, is dependent on what you're initial resources are...if u're running an old dual core processor and only have 4G of ddr3 ram, then the initial option with refind/dedicated partition might be the way to go.])
    
    What will u be using ubuntu for? 
    
    Have you created a liveusb and booted into ubuntu already ?! Are you familiar with linux?!