My apologies everyone. I overlooked an important thing twice when I was installing Linux Mint.  I have not asked it to erase the hard drive at installation.  Once I've done that, the Linux Mint installation has booted off hard drive. All that's left for me to do is to sort out the pfSense installation. Sorry for wasting your time.
I bought a refurbished Optiplex 9020 SFF to make it my pfSense box.  I disabled Secure Boot, enabled Legacy ROMs, changed Boot List Option from UEFI to Legacy, then changed boot order with USB Storage Device in first position and the hard drive second. TPM Security was unchecked and I left it that way. Exiting from the BIOS setup screen made the PC restart, then boot off USB where I had inserted the pfSense installation usb stick.  Installation to hard drive was easy and finished quickly.  At the end of the installation process, I selected reboot from the available options.  From here on the PC fails to boot.  Turning the PC off then on with the USB stick also fails to boot from USB.  The only way I can get back in was if I powered the unit off, short the RTC reset (RTCRST) jumper then power on. At this point the display advises "Invalid configuration information - please run SETUP program" and "Time-of-day not set - please run SETUP program".  I can update the date and time, but I don't know anything else that needs to change. Just in case it was the Boot List Option change that is not correct, I tried putting this back from Legacy to UEFI. Then did the whole second paragraph thing with no success.  I also installed Linux Mint in case the issue was with pfSense, however same result. I would appreciate any advice.
OptiPlex 9020 SFF refuses to boot OS