Check the soil for watering issues.
The soil should dry slightly between waterings.
If the soil is soggy and the plant is suffering from root rot, it could cause a soft spot in the trunk.
Look for any other signs of fungus or disease on the plant.
From your image, your plant seems actually quite healthy.
Your plant may heal the wound on it's own. Do not apply anything to the plant, allow air flow around the area to promote healing.