If you are on console, press up on your D-Pad while on foot (I believe it is Z on PC). This opens the build menu. Within that menu, you should be able to find the Portable Refiner.
Additional Source and information:
To build a Refiner, you have to open the Build Menu while on foot (press up on the d-pad on console). The Refiner is about the fourth one to the right.
You will need a Metal Plate and 30 oxygen to build the refiner. You will also need fuel to power it (like carbon).
I do not think the blueprint is locked behind a quest, but I do know it was needed to repair one of your ships broken components when you first start a new game with the Next update. When I played the other night, that is when I built my refiner. I assume when you reach that point for repairing your ship, the blueprint will be unlocked for you if it is indeed locked to begin with.