How to install pfSense using USB stick
Delete Key into BIOS
F11 Boot from USB stick
Download pfSense-CE-memstick-2.3.2-RELEASE-amd64.img.gz from https://www.pfsense.org/download/ , unzip it to get pfSense-CE-memstick-2.3.2-RELEASE-amd64.img put it into a USB stick using Win32DiskImage, connect it to Mini PC, install it!
Note that it is better to use USB2.0 stick and connect it to a USB2.0 on Mini PC.