To add a dependency on your Gantt, hover your mouse over the task you want to add the dependency until an arrow icon appears on the right side. Clicking on the arrow icon activates a blue dialog at the bottom asking you to select the tasks that will be dependent. Click on the desired tasks and then go to the bottom and click the green button called "add dependencies" - a line will link the selected tasks indicating the dependency.

To erase the dependency, you must open the task, click on "show details"  and click on the trash can icon.

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