cap aborted! SSHKit::Runner::ExecuteError: Exception while executing on host

Just write the command in console:

 ssh-add

If that don't work, write below command in console:

 ssh-add ~/.ssh/id_rsa

and then re-run the command (i.e. cap production deploy), It will work for sure.

Capistrano wasn't able to create the folder on ec2. Once I created the folder it worked like a charm.

So if you also have similar issues try creating the folder on ec2 and then execute the cap production deploy.

So what are the permissions for /var/www/my_app in your remote machine ? Make sure they are owned by the same user that you specified in this config option:

set :user, "mydeployuser"

If not I believe Capistrano defaults to the user you are using to run the 'cap' command on the client machine. Make sure that it has permissions to modify/create /var/www/my_app

Since you have:

set :use_sudo, true

it's possible that sudo may not be setup as passwordless sudo like suggested in the Cap 3 docs