I have a pie 5. I was able to get pifi working but noticed that the moment I change the wifi name or password pifi completely vanishes from being found as a wifi network. Had to reset, move the wifi dongle from one port to the other to get it to work again. Unsure if this is a known issue or if there is a fix.
Edit: restarted the process and accessed PiFi via the web. This time around I used a different password (more complicated) and it successfully applied.