I have some Graptoveria 'titubans' that I bought recently. I replotted them using cactus/succulent soil a few days ago. I put them indoor near the window with as much sun as possible and water them when the soil is completely dry. But for some reason the leaves of the plants start to turning soft and even falls (lost two leaves two days in a row). Can someone please help me.

It could be that the plant has to get used to its new location and growing condition especially after having been re-potted, then it is normal for it to loose some leaves.
Does its pot has a drainage hole? the potting soil looks quite water-saturated from your picture..

Welcome Rann! I think it's OK. Those leaves may have been "bumped" during replanting. The attachment of leaves to stem is kind of rigid so that a bump in the right degree could loosen/detach the leaf. And the lower leaves are the older leaves. Looking at your photo, the newer growing-in part is forming a bigger canopy over the lower leaves. It also looks like the some of the lower leaves are in the soil. Succulent leaves don't like to sit in soil as they will rot.