404 error when trying to open external link

Hi all,
I just installed GitLab on my ubuntu but failed to open the preset external link. Currently, nothing else is running on my conputer.
I followed the instruction 100% therefore I have no idea where the problem is.
I checked the gitlab.rb file and reconfigured the system but nothing changed.
I kindly ask for your help, how to diagnose this problem? Many thanks!