Support for the Ruby 2.4 series has ended. See here for reference.
# File rubygems/commands/build_command.rb, line 42 def execute gemspec = get_one_gem_name unless File.exist? gemspec gemspec += '.gemspec' if File.exist? gemspec + '.gemspec' end if File.exist? gemspec then spec = Gem::Specification.load gemspec if spec then Gem::Package.build spec, options[:force] else alert_error "Error loading gemspec. Aborting." terminate_interaction 1 end else alert_error "Gemspec file not found: #{gemspec}" terminate_interaction 1 end end