I presume this is a Database install (not ETL) so the first installation you can unfortunately get things wrong with a parameter and it will show this.
Take a look at this documentation and see if you entered everything correctly on the fixlet. https://www.ibm.com/support/knowledgecenter/en/SSTK87_9.5.0/com.ibm.bigfix.webui.doc/WebUI/Admin_Guide/c_before_running_fixlet_2556___i.html
There is another fixlet that can change the credentials so if you did something wrong you can try running that again.
If not, please contact support and raise a PMR as there are too many possible things to debug here. The activation of WebUI on an existing server can take a while but if this is constant there is something wrong. The support person will need to see the logs in the BESWebUI/WebUI/logs directory. There will also be information there if you want to see what is wrong as well.