Why do you believe that something is wrong? It looks somewhat shrivelled, but otherwise I can not see any obvious problems.
If the base turning yellowish and soft is because the cactus got too much water before you bought it or during your maintenance. It's may start to moldy. - Avoid putting water on it and don't spray. - Watering needed around 1-2x months and let dry until the next. - Full sun lighting.
