Is this weakness with tasks?
I have a tasks table which has sequential dependency. So the first task is what is driving others as being predecessor. I can mark my tasks as complete with a finish date but not quite the first on...
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It looks from your screen shot like you are trying to make this task a predecessor of itself, thereby creating a circular reference. The first task of a project shouldn't have any predecessors. At least that's the way we do it in our project-task app.