I spent many days trying to figure out how to get Linux working on my 2018 Mac Mini and 2008 Macbook Pro, which appear to be particularly difficult. ... I.e., if you want to install Linux, do NOT at least buy a 2018 Mac MIni nor a 2008 Macbook Pro, no matter how good Macs they may be!
Installing Linux on a newer Intel Mac (e.g. 2018)
Shut down. Press Cmd - Alt and bootup. Select Utilities - Terminal.
Disable system integrity protection: csrutil disable
Install Solus Budgie without problems by enabling Solus to install onto the entire disk.
Or, check this: https://getsol.us/articles/installation/disks/en/.
PS. Debian did not work. Ubuntu did not work. Clear Linux did not work etc. etc.
Extra: Bootloader
Download rEFInd. Open Mac Terminal.
Drag the refind-installl file from the rEFInd directory onto the Terminal.
Install Linux. Reboot pressing Alt.
Sometimes Linux will overwrite rEFInd and you may then need to reinstall rEFInd.