Hi,
See this post: Upgrading from 13.12 to 14.0
Unicorn has been deprecated since 13.10 so this is why you have issues. Remove every reference to unicorn that has been configured manually in your /etc/gitlab/gitlab.rb and run:
gitlab-ctl reconfigure
if no entries for unicorn exist, then on your current version it will use unicorn. Then when you upgrade, since there are no references to unicorn it will automatically switch over and start using puma which replaced unicorn.
The second post on that link is by me, which explained what to do with the config files. Read through the thread and you should get past your problem.