I am trying to install it on a rasberry pi running Ubuntu20.04.3 LTS and getting "tool arm-none-eabl-gcc is not available for your operating system". I tried installing it on my desktop linux environment with ubuntu 20.04.2 installed and the apollo3 board core installed just fine.
The situation is explained in this bug report in SparkFun's repository for this project:
The matter of adding tool definitions for the ARM Linux host versions of the tool dependencies to the package index file is trivial. They even have such tool definitions for other cores in their own package index to use as a reference.
The tricky thing is that the tools must be compiled for the ARM Linux (and ARM Linux 64 bit if you want to support that as well) hosts. The tool that was compiled for the x86 Linux host will almost surely not work on the Raspberry Pi. That has the potential to be a very complex process. Or maybe it's easy, or maybe someone else has already done it and it's only a matter of finding where they have shared those binaries. But it doesn't appear that SparkFun has any intention of doing it, so I think it's up to the Arduino community to make it happen.