Here's a picture of a messy amateur installation:
Look closely and find the power feeder conductors entering the panel on the bottom. The white rises up to the right and terminates in the "neutral"
bus lug on the top right. The black also rises up to the right and terminates in the left "line"
bus lug on the left. The red wire rises up to the left and terminates in the "line"
bus lug on the right - just below the neutral lug. Finally, the ground rises left and terminates in a separate ground bar
that is secured directly to the back of the upper left side of the panel box in pre-tapped holes for this purpose.
Since you are supplying this panel from your home's panel, you MUST keep the neutral and ground feeders separated in this panel
(even though they will both probably originate from the neutral bar in your home's service panel). You must NOT install the long "bond" screw through the neutral bar in to the panel box. If the long bond screw is already installed, you must REMOVE it
(usually this screw it shipped loose in a bag with a tag on it).
When the picture was taken, the flash reflected back making the right side of the neutral bar difficult to see - so I can't determine if the screw is installed there or not. When connecting circuits to this panel, the hot wires will of course go to the circuit breaker terminals, but the neutral and ground wires must not be mixed. The ground wires (and ONLY the ground wires) must terminate in the ground bar. The neutral wires (and ONLY the neutral wires) must terminate in the neutral bus bar.
The person that wired this panel has mixed them. This would be a dangerous situation if the the long bond screw mentioned above is installed in the neutral bar to bond it to the panel box.
I hope this helps and good luck. Please rate my reply. Thanks.