Assuming you have a 12VDC In 120V AC out 3000W inverter.
LUP indicates battery voltage is low, so firstly make sure:
1. You 12V battery is good, it's large enough a minimum of 75AH and probably much larger is recommended and it's fully charged.
2. Verify you are using the correct size copper wires from battery to the Inverter DC INPUT(s). If you keep wire short the wire size can be smaller, but if the DC input wires are longer then you will need to use thicker copper wires.
At MAx load you would be taking 250 AMP from battery, so for a 5% Volatge drop of .6 V for a 10 ft length of cables from battery to Inverter you would need to use a 1AWG copper cable.
OLP is a overload condition, which means you may have to many high powered AC powered things connected to the Inverters AC outlets or maybe something that has large start up surge, ie devices large electric motors.
Or it could be all due to a faulty Inverter.