The battery is weak and should be replaced, with a charged one or new one.
Defective alternator.
Charge the BATTERY
Your alternator is dead and needs replacing.
lack of fuel
Your car has to perform a drive cycle in order for the self diagnostics to complete it's test. If your car had a dead battery recently or the battery was disconnected recently it won't pass emissions until it has performed a drive cycle.
Are your battery cable connections clean and tight ?
Must be an electrical short.
It can be. but generally wont be unless its your first payment.
safety reasons
that's the point of repossession. you cant pay, they take it away.
You can run an engine in/out of a car without a transmission. Car wont move and engine wont be mounted properly. So i suggest not to run an engine without a transmission unless you know what you are doing.
bad battery, and bad alternator not keeping it charged - also bad connections to the battery or loose wire connections on the alternator.