Welcome to the asdf-ruby wiki!

Mint does not come with all of the required build tools out of the box. build-essential is a meta-package that installs all of the required build packages. The rest of the packages are Ruby dependencies.

Before asdf install ruby x.x.x, run the following:

$ sudo apt install build-essential
$ sudo apt install zlib1g-dev libssl-dev libreadline-dev libgdbm-dev

If you installed ruby before installing the prerequisites, make sure you uninstall and reinstall.

$ asdf uninstall ruby x.x.x
$ asdf install ruby x.x.x

Note: I'd installed Ubuntu 18.04.1 LTS on my Windows 10 Pro machine (build 1803) using VMWare Workstation 15 Pro. These were the steps I followed to successfully use asdf to install Ruby 2.5.3.

1. Install asdf
2. Install the asdf ruby plugin
3. Install GCC-6. (Older GCC version required to install Ruby because GCC-7 causes problems.)
4. Run sudo apt install build-essential. (Note that this does install GCC-7 which we don't want though. Oh well.)
5. Run sudo apt install -y libssl-dev libreadline-dev zlib1g-dev to install additional libraries that are required.
6. Run CC=/usr/bin/gcc-6 asdf install ruby 2.5.3 to use asdf to install Ruby with GCC-6.