For a task to appear in the predecessor drop-down, it must have both an Estimated Start Date and Estimated End Date.
However, if you have a string of dependent tasks, it is not necessary to provide an estimated start and estimated end date for all tasks prior to setting the dependencies. All you need to do is provide an estimated start and estimated end date on the first task in the series. Then, open the 2nd task in the series (by clicking on its Summary link in a task list), select the “Dependencies” tab and choose the 1st task as its predecessor . AceProject will suggest dates for your current task (2nd task in the series) based on the Estimated End Date of the chosen predecessor. You may, of course, accept or make changes to the suggested dates.
You may then repeat the process from the 3rd task in the series to the last.
Since the introduction of the enhanced Task Gantt chart, it is easier and more visual to add dates and dependencies dynamically within the Gantt chart.