Why Can't I add another remote to a merlin mt60p

If you purchase additional transmitters, the garage door
opener must be programmed to accept the new remote
code.
Program the Receiver to Match Additional Transmitter
Codes:
Using the orange "LEARN" Button:
1. Press and hold down the transmitter button you wish to
program to the opener. The orange LED will flash to
indiciate it is receiving signal from the transmitter.
2. Press and release the "LRN" button.
3. The sourtesy LEDs will flash once.
Ensure the door is clear of obstruction, then test the
transmitter.
Using the Motion Detecting Control Panel
(optional accessory):
1. Press and hold the button on the hand-held remote that
you wish to operate your garage door (1).
2. While holding the remote button, press and hold the
LIGHT button on the Motion Detecting Control Panel (2).
3. Continue holding both buttons while you press the push
bar on the Motion Detecting Control Panel (all three
buttons are held) (3).
4. Release buttons when the opener light flashes. It has
learned the code. If the light bulb is not installed, two
clicks will be heard (4).
Now the opener will operate when the transmitter push
button is pressed. If you release the transmitter push button
before the opener light flashes, the opener has not learned
the code.
To Erase all Transmitter Codes:
To deactivate any unwanted remote, first erase all codes:
Press and hold the orange "learn" button on opener until
the learn indicator light goes out (approximately 6 seconds).
All previous codes are now erased. Reprogram each remote or keyless entry you wish to learn.
http://gomerlin.co.nz/getattachment/2949a1dd-74ab-4576-850d-4f40de28fbe3/mt60p-installation-guide part 27 from installation manual page 15