What green light are you talking about? All the IKEA devices/sensors what I know has red LEDs.
And pairing is the following. Add device in the app, IKEA/Motion sensor… when starts to search for it you press 4 times the button on back of the motion sensor, then on the front the red LED starts to flash in a breathing pattern, until it flashes once when connected. Then the app will show in a minute or two that the device is paired.
If when pressing 4 times the button will not start the LED on the front, or the other other two button on the back are not changing the position of the settings, then you have a depleted battery.
Knowing that there is an ongoing issue with IKEA devices depleting quickly (1-2 days max), then I would think that the motion sensor is hitting that bug as well.
https://community.smartthings.com/t/ikea-5-button-remote-killing-batteries