digital ocean support response:

This is because when a new version of your app is deployed, the directory is empty.
Fortunately, you can resolve this by creating a symlink to a shared directory.
You can do this by editing the deploy.rb file, changing the following line

set :linked_dirs, %w{bin log tmp/pids tmp/cache tmp/sockets vendor/bundle public/system}

to:
set :linked_dirs, %w{bin log tmp/pids tmp/cache tmp/sockets vendor/bundle public/system

and its working now