I brought this from nursery last week, repotted in Top soil and water gently. Today I started noticing a yellow leaf. Can someone help me to understand what's wrong with this? I don't want this to die.
A single yellow leaf at the base of the plant is nothing to worry about - it will probably fall off on its own in time, or go brown and dry, when you can remove it. Just make sure you don't overwater - water only when the surface of the soil feels dry to the touch, but not so dry it's shrunken from the sides of the pot. Water thoroughly using about half a litre to a litre of water, let it drain down freely and don't water again until the soil feels dry to the touch.
The only other thing is, it would have been better to use proper potting soil, not topsoil, which may not be sterile.
The most common causes of problems with these plants is not enough light and too much water leading to root rot.
As Bamboo mentions topsoil is not the best choice for this plant as a free draining soil is better in indoor conditions.
I suggest moving it towards the light and checking for spider mites on a weekly basis. They are common for this plant and look like tiny grains of salt on the underside of the leaf. Control is wiping the plant down with a cloth soaked in dish soap and water. Do this three times at five day intervals if you see spider mites of their webs
It is also advised to take the picture from an angle where the whole stem is visible down to the soil. A sudden death of the top part may result from basal rot, be it from overwatering or just compacted soil having difficulty draining, or a pathogen. Bamboo is correct about the yellowing of the bottom leaves: the old leaves turn yellow and fall-off first. The alarming sign is when you have massive yellowing. It also starts at the bottom leaves but may spread more quickly to the top. When the stem base rots, you see the stem shriveling, like a deflating firefighter's hose and it becomes very wrinkled. Ironically, underwatering and overwatering may cause the same effect, which may encourage many people to overwater their plants. However, underwatering is much easier and safer to rectify. Always check the base, and if you can it is advised to check the roots. A dead stem can be cut above the rot zone and the tip is rooted as a cutting. The porous property of the clay allows the soil to dry more quickly and leaves a wider margin for errors. topsoil is much too compact as it contains much fine grade particles. A coarser potting medium is advised.