Hi Angelina, your schefflera seems to be okay.. Sometimes symptoms of underwatering and overwatering looks the same, with leaves going brown and dropping, or maybe the plant is just dropping older leaves.
Observe it a little more and since you have this plant indoors, the soil needs to be moist and allowed to dry out a bit. I grow my schefflera outdoors in a container year round and it is very hot and dry here. It is rather drought tolerant once growing very well, so just be mindful of watering, since you say you are using Miracle gro potting soil.
