Celeriac is not listed on the SIGHI Food Compatibility List. It is not on any list as being high in histamine, but that could be because it is an odd enough vegetable that it wasn't considered. I seem to do fine with it. Test it carefully if you are unsure whether or not it will work for you.
I live at 7000 feet so my pressure cooking times are longer. If you live at a lower altitude then try cooking for 15-17 minutes. You need the celeriac to be really tender so it will blend smoothly.