Problems encountered in installing Gitlab

I had git installed on my ubuntu 16.04 server and I installed gitlab on top of it unknowingly.

Running handlers:
There was an error running gitlab-ctl reconfigure:

account[GitLab user and group] (gitlab::users line 28) had an error: Mixlib::ShellOut::ShellCommandFailed: linux_user[GitLab user and group] (/opt/gitlab/embedded/cookbooks/cache/cookbooks/package/resources/account.rb line 51) had an error: Mixlib::ShellOut::ShellCommandFailed: Expected process to exit with [0], but received '8'
---- Begin output of ["usermod", "-s", "/bin/sh", "-d", "/var/opt/gitlab", "git"] ----
STDOUT: 
STDERR: usermod: user git is currently used by process 1238
---- End output of ["usermod", "-s", "/bin/sh", "-d", "/var/opt/gitlab", "git"] ----
Ran ["usermod", "-s", "/bin/sh", "-d", "/var/opt/gitlab", "git"] returned 8

i have changed accounts in gitlab.rb,but invalid。

user['username'] = "gitlab" 

user['group'] = "gitlab"