漏れは、Windows 10,Ruby 2.5 で、
Run ‘ridk install’ to install MSYS2 and development toolchain. も、チェックしていないし、
MSYS2 の開発ツールと、MINGW のコンパイラもインストールしていないから

Rubyの環境構築（Windows10編）
https://www.kenschool.jp/blog/?p=3898

エラーログ

C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/ruby_installer/runtime/msys2_installation.rb:63:in `iterate_msys_paths': undefined method `split' for nil:NilClass (NoMethodError)\r\n\t

from C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/ruby_installer/runtime/msys2_installation.rb:76:in `msys_path'\r\n\t

from C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/ruby_installer/runtime/msys2_installation.rb:89:in `mingw_bin_path'\r\n\t

from C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/ruby_installer/runtime/msys2_installation.rb:99:in `enable_dll_search_paths'\r\n\t

from C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/ruby_installer/runtime/singleton.rb:27:in `enable_dll_search_paths'\r\n\t

from C:/Ruby25-x64/lib/ruby/2.5.0/rubygems/defaults/operating_system.rb:3:in `<top (required)>'\r\n\t

from C:/Ruby25-x64/lib/ruby/2.5.0/rubygems.rb:1383:in `require'\r\n\t

from C:/Ruby25-x64/lib/ruby/2.5.0/rubygems.rb:1383:in `<top (required)>'\r\n\t

from <internal:gem_prelude>:2:in `require'\r\n\t

from <internal:gem_prelude>:2:in `<internal:gem_prelude>'\r\n