Unable to install gem - Failed to build gem native extension - cannot load such file -- mkmf (LoadError)

There are similar questions:

• `require': no such file to load -- mkmf (LoadError)
• Failed to build gem native extension (mkmf (LoadError)) - Ubuntu 12.04

Usually, the solution is:

sudo apt-get install ruby-dev

Or, if that doesn't work, depending on your ruby version, run something like:

sudo apt-get install ruby1.9.1-dev

Should fix your problem.

Still not working? Try the following after installing ruby-dev:

sudo apt-get install make

For WSL (Windows Subsystem for Linux) you need install build-essential package:

sudo apt install build-essential

Just finished a 2 hour wild goose chase trying to solve this. None of the posted answers worked for me. Im on a Mac (Mojave Version 10.14.6, Xcode Version 11.3).

It turns out the ruby file headers were missing so i had to run open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg

That didnt work for me at first because the version of CommandLineTools i had installed did not have the "Packages" folder. So i uninstalled and reinstalled like this:

rm -rf /Library/Developer/CommandLineTools

xcode-select --install

Then i ran the previous command again:

open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg

After install the error was fixed!