これまで遭遇したエラーについて簡単にまとめていく。
今回は、

AWSへのデプロイ時のエラー

前提条件

Capistranoを使用したデプロイでのエラー

遭遇したエラー(一部抜粋)

`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)

解決法

以下のgemを追加

gem 'ed25519'
gem 'bcrypt_pbkdf'

bundle installの実行
以上で終了