When you start TigerVNC server by following command:

vncserver

then it will automatically bind to port 5901.

When you want to start the server on 5900 then you need to type following command:

vncserver :0

This looks a little bit mysterious, but if you dig through the documentation you might find that there is the following mapping:

:0 -> 5900
:1 -> 5901
:2 -> 5902
:3 -> 5903
...

Now it’s clear that when you omit the parameter, it will default to code :1 which is mapped to the port 5901.