Hi
Depends on which model of RAT (Ram Air Turbine) is installed.
The old type RAT will stall when the gear is lowered due to the disturbed airflow from the wheel assembly, and landing is made on batteries only. Therefore it is not recommended to start the APU as it uses the batteries to start which should be conserved for the approach with gear down.
The new RAT doesn't stall on gear down and therefore the batteries are not so essential and can be used for APU starting.
ECAM knows which RAT is installed and therefore prompts the pilots to start the APU if it is the new RAT, otherwise this ECAM action line is missing.
Hope this helps
Pro Spin