As you update scheduling information for an automatically scheduled task, such as task duration and start and finish dates, ...

As you update scheduling information for an automatically scheduled task, such as task duration and start and finish dates, Microsoft Project automatically calculates other tasks that have dependencies on the task. The task displayed in the initial error message is causing at least one other task to be rescheduled to a different date due to a task dependency (link). However, the other task cannot honor both its own scheduling conditions and the predecessor task's dependency without creating negative total slack (float) in the schedule. Most often, this is due to an inflexible constraint on this task, such as a Must Start On constraint type.