Cannot create systemd script for Puma

I found the problem and changed the ExecStart as mentioned below and it worked like a charm:

ExecStart=/home/username/.rbenv/shims/bundle exec puma -e production -C ./config/puma.rb config.ru
PIDFile=/home/username/appdir/shared/tmp/pids/puma.pid

bundle should be taken from the rbenv shims and also the puma's config file (config/puma.rb) and application's config file (config.ru) can be given in relative path.