Hi,
have you flashed the application firmware after you flashed the bootloader via bossac?
this will be the case when you power it up and only the green led is lit.
see https://osmocom.org/projects/simtrace2/wiki/Flashing#DFU
or simply:
sudo dfu-util --device 1d50:60e3 --cfg 1 --alt 1 --reset --download simtrace-trace-dfu.bin
afterwards you will see both leds (red and green) lit when plugging it in.
kind regards