Hi Zhou,
The KKT matrix problems are quite often discussed in detail on the forum. Please try searching for the error message next time - you may find the answer right away.
But it typically indicates that you have a problem with balance in the system. For example, one of the degrees of freedom is not supported by muscles or reaction forces or environment, or supporting muscles are too weak, or accelerations of the relevant segments are too high for the muscles to generate such motion. I don't know your exact problem - so cannot help here. Try to think about your motion and what should govern this motion.
An example could be that if you try to model a seated human without having the chair under the human - muscles would not support this posture and you would get a KKT error. Adding a large extension joint torque at the knee could help resolving this. Something similar might help in your case - maybe your exoskeleton creates an instability. Can human do this motion without exoskeleton? It is up to you to analyze your particular problem.
Kind regards,
Pavel