Odoo doesn't have any built-in functionally to measure tasks progress and based on it project progress.
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All you can do is create stage named "Complete" name move task to this stage when it completed. it will not reflect anything on the parent project. so you could not say the project is still running or completed unless you open the project and see all the tasks are in the "Complete" stage.

there is OCA module  try to add this functionality (it create stages for a project too like tasks)
https://github.com/OCA/project/tree/12.0/project_stage_state
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Note: there is a progress bar in task timesheet but not related to the stage (it indicate progress timesheet entry vs planned hours)

Hi! 😃

I find how to set a task as closed!

You have to enable Folded in Kanban. And thats all :-D

There's no answer to this question?  Does that mean there is no way to complete a task? That seems like a pretty obvious oversight. Or perhaps I'm missing something in the workflow...

Anyone have thoughts?