You are following the steps to upgrade GitLab from 7.14.3 to 8.4.2 but really, you are doing a major upgrade (7.14.3 to 8.13.6) and with a wrong documentation. I recommend you separate the process into two phases, first you should upgrade GitLab from 7.14.3 to 8.4.2 following this documentation and later upgrade GitLab from 8.4.2 to 8.13.6.
I hope this information is useful for you. If you have any other question, please do not hesitate to let us know.
Carlos R. Hernández