Your primary line is probably clogged up with debris. If you want to try it, you can take off one of the side panels on the unit, and see whatís blocking the drain line. Make sure you turn off the unit first. You can use a shop vac to blow out the line (from the outside), but I would first try and blow the clog up to the unit, so you donít lodge the debris into the line, by sucking it at first.
Make sure you take care of this problem as soon as you can, because if the second line gets clogged up the water will over flow and soak your drywall, (if your unit is in the attic).