From my experience Schefflera arboricola are an easy plant to keep when conditions remain stable. Sudden changes in position can cause for leaf drop. Keep your plant in a bright, warm location and aim to maintain damp but not wet soil.
For optimum conditions keep your plant between 15-24 degrees. The leaves will soon begin to drop if its exposed to a lower or higher temperature. Or if the plant is suddenly moved to a different temperature. They will tolerate a wider temperature range of 12-26 degrees.
Provide your Umbrella tree with a bright position. This position can receive some sunlight. Sudden changes in position that has a differing in brightness can result in the leaves dropping. Occasionally rotate the pot of your plant to even out the growth otherwise it will begin to arc toward the light source.
Water your umbrella plant whenever the soil begins to dry out. Aim to keep the soil damp to the touch but not wet otherwise the roots will rot.
Use a peat alternative such as composted bark that is mixed with a potting soil. The peat alternative will help to retain some moisture and create an airier finer soil. Add grit to make the mix to ensure the soil can drain easily and won’t become waterlogged.