Diagnosis
The infrared receiver window on the AC unit itself is probably blocked by dust or grime, or you've got some strong ambient light (like direct sunlight) flooding the sensor and drowning out the remote's signal.
How to Fix It
First, find the small, dark plastic IR window on the control panel of the AC. Wipe it clean with a dry, soft cloth. Then, try the remote from up close (3-5 feet away), pointing directly at that spot. Make sure no bright lights or the sun are shining right on it. Test it.
If That Didn't Work
The remote itself might have a faulty circuit. Try the classic test: point your phone's camera at the front of the remote and press a button. You should see a faint purple light flashing on your phone screen if the IR LED is working. If you see nothing, the remote is dead.
Heads Up
These universal learning remotes rarely work with these portable ACs because the codes are proprietary. You're usually stuck ordering a replacement directly from the manufacturer if the remote is truly gone.