"fatal: Authentication failed" error when deploying (private repo) - ruby-on-rails

Question

I am trying to deploy my Ruby on Rails app to the production server hosted on Digital Ocean. There is no problem occurs when I set my repository to "Public", but when I set my repository on "Private" I got the "fatal: Authentication failed" error:
SSHKit::Runner::ExecuteError: Exception while executing on host 178.62.16.69: git exit status: 128
git stdout: remote: Invalid username or password.
fatal: Authentication failed for 'https://github.com/ryzalyusoff/xxxxxx.git/'
git stderr: Nothing written
SSHKit::Command::Failed: git exit status: 128
git stdout: remote: Invalid username or password.
fatal: Authentication failed for 'https://github.com/ryzalyusoff/xxxxxx.git/'
git stderr: Nothing written
Tasks: TOP => git:check
(See full trace by running task with --trace)
The deploy has failed with an error: Exception while executing on host 178.62.16.69: git exit status: 128
git stdout: remote: Invalid username or password.
fatal: Authentication failed for 'https://github.com/ryzalyusoff/xxxxxx.git/'
git stderr: Nothing written
So, what should I do so that I will be able to deploy to my production server without needed to set my repo on "Public" ?
Thanks!
------------------------- UPDATE 1 ----------------------------
I already generated the public key and put it on my github account. I have confirmed this by tested it by using the command below:
ssh -T git#github.com
------------------------- UPDATE 2 ----------------------------
I have changed my set :repo_url to ssh, but still the error persist. Here is my deploy.rb file:
# Change these
server '178.62.16.69', port: 22, roles: [:web, :app, :db], primary: true
set :repo_url, 'git#github.com:ryzalyusoff/xxxxxx.git'
set :application, 'xxxxxx'
set :user, 'deploy'
set :puma_threads, [4, 16]
set :puma_workers, 0
# Don't change these unless you know what you're doing
set :pty, false
set :use_sudo, false
set :stage, :production
set :deploy_via, :remote_cache
set :deploy_to, "/home/#{fetch(:user)}/apps/#{fetch(:application)}"
set :puma_bind, "unix://#{shared_path}/tmp/sockets/#{fetch(:application)}-puma.sock"
set :puma_state, "#{shared_path}/tmp/pids/puma.state"
set :puma_pid, "#{shared_path}/tmp/pids/puma.pid"
set :puma_access_log, "#{release_path}/log/puma.error.log"
set :puma_error_log, "#{release_path}/log/puma.access.log"
set :ssh_options, { forward_agent: true, user: fetch(:user), keys: %w(~/.ssh/id_rsa.pub) }
set :puma_preload_app, true
set :puma_worker_timeout, nil
set :puma_init_active_record, true # Change to false when not using ActiveRecord
## Defaults:
# set :scm, :git
# set :branch, :master
# set :format, :pretty
# set :log_level, :debug
# set :keep_releases, 5
set :whenever_identifier, ->{ "myapp_#{fetch(:stage)}" }
namespace :puma do
desc 'Create Directories for Puma Pids and Socket'
task :make_dirs do
on roles(:app) do
execute "mkdir #{shared_path}/tmp/sockets -p"
execute "mkdir #{shared_path}/tmp/pids -p"
end
end
before :start, :make_dirs
end
namespace :deploy do
desc "Make sure local git is in sync with remote."
task :check_revision do
on roles(:app) do
unless `git rev-parse HEAD` == `git rev-parse origin/master`
puts "WARNING: HEAD is not the same as origin/master"
puts "Run `git push` to sync changes."
exit
end
end
end
desc 'Initial Deploy'
task :initial do
on roles(:app) do
before 'deploy:restart', 'puma:start'
invoke 'deploy'
end
end
desc 'Restart application'
task :restart do
on roles(:app), in: :sequence, wait: 5 do
invoke 'puma:restart'
end
end
before :starting, :check_revision
after :finishing, :compile_assets
after :finishing, :cleanup
after :finishing, :restart
end
THE ERROR

Answer 1

Capistrano assumes you will SSH key forwarding to authenticate to Git. This means the key that you use on your local machine will be the one that your server uses as well when you perform a cap deploy.
Of course, this will only work if you are using SSH to connect to Git. Based on your error messages, it looks like you are not. You seem to be using the https: URL instead of the SSH one.
Change your :repo_url to the SSH style and you should be good.
E.g.
set :repo_url, "git#github.com:ryzalyusoff/xxxxxx.git"

Answer 2

You need to create an ssh key and provide github with your public key.
$ssh-keygen -t rsa
Copy the text *.pub to your clipboard and add it to your github account.