Have you been able to complete the installation yet? What version of Tableau Server is this occurring in?
There are a couple steps that need to be followed if your doing an upgrade to Tableau Server from 2018.1 or lower to 2018.2.
Hope this helps!
Yes, once I used port 80 as the gateway port it worked. The documentation made it sound like you had to use whatever port the admin website uses. In this case, 8850.