slowhttptest showing up as service unavailable, but is inaccurate service still up and running

Backstory: I’ve got a small contract with a website; they have given me permission to stress test their website, testing for DOS attacks. I have some experience in the field, but I am no omniscient hacker.

Problem: I’m running kali linux in a VM, with plenty of computing power allocated for a slowhttptest. I’ve tried the different tests that the program has to offer, and it appears to slow the website to some degree, temporarily, but my slowhttptest is coming up as service unavailable (it doesn’t connect from the VM browsers). However, when I try to visit from any other browser the site loads a bit slower at best (no success in a temporary DOS). I’m wondering if it is some kind of filtration or if (more likely?) I need to run through a proxy or something of the sort. Here is an example of the command I am running:

slowhttptest -c 100 -X -g -o slow_read_stats -r 15 -w 512 -y 1024 -n 10 -z 32 -k 3 -u {weburl} -p 10