The current runner (ubuntu-undefined-x64) was detected as self-hosted because the platform does not match a GitHub-hosted runner image (or that image is deprecated and no longer supported). In such a case, you should install Ruby in the $RUNNER_TOOL_CACHE yourself, for example using https://github.com/rbenv/ruby-build You can take inspiration from this workflow for more details: https://github.com/ruby/ruby-builder/blob/master/.github/workflows/build.yml $ ruby-build 2.6 /opt/hostedtoolcache/Ruby/2.6/x64 Once that completes successfully, mark it as complete with: $ touch /opt/hostedtoolcache/Ruby/2.6/x64.complete It is your responsibility to ensure installing Ruby like that is not done in parallel.