It is not advisable to place boxes or any other objects directly in front of a split AC compressor while it is operational. The compressor of an air conditioner requires proper airflow to function efficiently and dissipate heat. Blocking the airflow by placing objects in front of the compressor can restrict the air intake and cause the compressor to overheat.
Overheating can lead to reduced cooling capacity, increased energy consumption, and potentially even damage to the compressor. It may also trigger the air conditioner's safety mechanisms, such as automatic shutdown, to protect the unit from overheating.
If you need to keep the area safe for kittens or any other reason, it's best to find an alternative location for the boxes that does not obstruct the airflow around the compressor. Ensure there is enough space for proper ventilation to maintain the optimal performance of the air conditioner.