DIAGNOSIS This usually means the internal condensate drain pan is overflowing because the unit isn't tilted correctly. Portable ACs need a slight backward tilt so water drains out the exhaust hose along with the hot air.
FIX First, check the unit's level. The back (where the hose attaches) should be slightly *lower* than the front. Use shims under the front wheels if you have a perfectly level floor. Second, make sure the exhaust hose is fully snapped into its port and hasn't come loose, creating a gap. Finally, straighten any sharp kinks in the hose—a restricted airflow reduces the force that pulls moisture out.
IF THAT DIDN'T WORK In very humid conditions, these units can produce more water than the exhaust can handle. You might need to manually drain it. Unplug the unit, slide out the bottom drain plug (usually on the back or side), let the water drain into a pan, then replace the plug firmly.
HEADS UP Don't run it with a big puddle underneath. Water can seep into the electronics or the float sensor, causing new error codes or a short circuit.