Depending on the species of succulent they can be hit and miss with cutting propagation success. Rot can be killer of youngly rooting/rooted cuttings. If the rot has already begun in major areas the best you can do is abandon ship and restart with rot-free cuttings (if there’s enough plant material you can definitely just recut and use what you already have). To prevent rot in a succulent cutting (this does not apply to non succulents like impatiens and coleus) the best you can do is mark sure its being grown/rooted somewhere warm, and that the moisture levels are kept more on the dry side than the wet side (so do water, but do so sparingly).

Not enough moisture is another possible candidate but less likely, and not enough sunlight could be a likely candidate too but it all depends on how the plant is dying.

My apologies if my answer sounds/feels vague, it can be like grasping straws when there’s no visual context to go with the information for plant pathology.