対応

以下を追加してbundle installで解決

group :development do
  # 〜略〜
  gem 'ed25519'
  gem 'bcrypt_pbkdf'
end

エラー

$ bundle exec cap production deploy SERVER_IP=IPアドレス
#<Thread:0x00007fe0069f1bc0@/Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/sshkit-1.21.4/lib/sshkit/runners/parallel.rb:10 run> terminated with exception (report_on_exception is true):
Traceback (most recent call last):
        24: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/sshkit-1.21.4/lib/sshkit/runners/parallel.rb:12:in `block (2 levels) in execute'
        23: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/sshkit-1.21.4/lib/sshkit/backends/abstract.rb:31:in `run'
        22: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/sshkit-1.21.4/lib/sshkit/backends/abstract.rb:31:in `instance_exec'
        21: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/capistrano-rbenv-2.2.0/lib/capistrano/tasks/rbenv.rake:10:in `block (3 levels) in <top (required)>'
        20: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/sshkit-1.21.4/lib/sshkit/backends/abstract.rb:61:in `test'
        19: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/sshkit-1.21.4/lib/sshkit/backends/abstract.rb:148:in `create_command_and_execute'
        18: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/sshkit-1.21.4/lib/sshkit/backends/abstract.rb:148:in `tap'
        17: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/sshkit-1.21.4/lib/sshkit/backends/abstract.rb:148:in `block in create_command_and_execute'
        16: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/sshkit-1.21.4/lib/sshkit/backends/netssh.rb:130:in `execute_command'
        15: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/sshkit-1.21.4/lib/sshkit/backends/netssh.rb:177:in `with_ssh'
        14: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/sshkit-1.21.4/lib/sshkit/backends/connection_pool.rb:63:in `with'
        13: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/sshkit-1.21.4/lib/sshkit/backends/connection_pool.rb:63:in `call'
        12: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/net-ssh-6.1.0/lib/net/ssh.rb:255:in `start'
        11: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/net-ssh-6.1.0/lib/net/ssh/authentication/session.rb:72:in `authenticate'
        10: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/net-ssh-6.1.0/lib/net/ssh/authentication/session.rb:72:in `each'
         9: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/net-ssh-6.1.0/lib/net/ssh/authentication/session.rb:86:in `block in authenticate'
         8: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/net-ssh-6.1.0/lib/net/ssh/authentication/methods/publickey.rb:19:in `authenticate'
         7: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/net-ssh-6.1.0/lib/net/ssh/authentication/key_manager.rb:111:in `each_identity'
         6: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/net-ssh-6.1.0/lib/net/ssh/authentication/key_manager.rb:250:in `load_identities'
         5: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/net-ssh-6.1.0/lib/net/ssh/authentication/key_manager.rb:250:in `map'
         4: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/net-ssh-6.1.0/lib/net/ssh/authentication/key_manager.rb:257:in `block in load_identities'
         3: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/net-ssh-6.1.0/lib/net/ssh/key_factory.rb:42:in `load_private_key'
         2: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/net-ssh-6.1.0/lib/net/ssh/key_factory.rb:51:in `load_data_private_key'
         1: from /Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/net-ssh-6.1.0/lib/net/ssh/key_factory.rb:200:in `classify_key'
/Users/username/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/net-ssh-6.1.0/lib/net/ssh/authentication/ed25519_loader.rb:21:in `raiseUnlessLoaded': OpenSSH keys only supported if ED25519 is available (NotImplementedError)
net-ssh requires the following gems for ed25519 support:
 * ed25519 (>= 1.2, < 2.0)
 * bcrypt_pbkdf (>= 1.0, < 2.0)
See https://github.com/net-ssh/net-ssh/issues/565 for more information
Gem::LoadError : "ed25519 is not part of the bundle. Add it to your Gemfile."
(Backtrace restricted to imported tasks)
cap aborted!
NotImplementedError: OpenSSH keys only supported if ED25519 is available
net-ssh requires the following gems for ed25519 support:
 * ed25519 (>= 1.2, < 2.0)
 * bcrypt_pbkdf (>= 1.0, < 2.0)
See https://github.com/net-ssh/net-ssh/issues/565 for more information
Gem::LoadError : "ed25519 is not part of the bundle. Add it to your Gemfile."

Tasks: TOP => rbenv:validate
(See full trace by running task with --trace)

参考

[https://qiita.com/Tosh39/items/7bac004d821299cc4f6b:embed:cite]