An unhandled lowlevel error occurred. The application logs may have details
This is because you haven't set your secret key correctly. Double check your config/secrets.yml file: It should be something like this:
production: secret_key_base: <%= ENV["SECRET_KEY_BASE"] %>
Then in your droplet, you can run bundle exec rake secret to get your secret key. There are options like dotenv which is a useful gem that loads the contents of a .env file into ENV.