No joy I’m afraid 
I created the directory /etc/systemd/system/dump1090-fa.service.d (no dump1090-fa.service.d directory in system) and then in the new directory created the bias-t.conf file with ‘ExecStartPre=/home/pi/rtl_biast/build/src/./rtl_biast -b 1’ as the only line in the file - rebooted and no voltage on the dongle output.
I tried running ‘/home/pi/rtl_biast/build/src/./rtl_biast -b 1’ from the command line and the error message came up as my first post.
I want to place the amp in my attic where the cable comes in, so would be great to get this biast option working.
Any other ideas??