From the manual::

Err 2 shows on screen; the second sensor did not detect the passage of the bullet. Poor light or poor alignment.

I use the F1 indoors with a 21 LED Puck light below each sunscreen (held with magnets); quite consistent, but when the batteries run low Err2 comes up. Chrony on a tripod, gun rested and sighted onto a target to maintain the right height above both sensors.