I bought a zz plant three weeks ago and had to repot it because it was in a really small pot. I put some peat moss in a larger pot and put the plant in, then added some peat moss. Finally, I watered it, but only that time. Then, gradually, the leaves turned yellow. I noticed that the stems have rotted and there are some mold lines on it. The plant died, unfortunately, and I had to throw it a way. By the way, one stem remained green, which was broken, while the other turned yellow earlier. I really like zz plants and want to get another one but I need some help to keep it safe and growing.
Your plant was likely placed into a pot to large for it's root system.
This led to overwatering and to much water for the rhizome.
A ZZ plant can go months without water.
Here is a link to refresh you on the care requirements.