To mark a task as completed, go directly to the desired task or first to the task overview Organization - Tasks - Overview.
In the task overview (tasks desktop) you can see that a tick can be placed in front of each project. If you or one of your employees has completed a task, this check mark must be set. Only then is the task considered completed. (More information on the task overview is available here)
As soon as you tick the task, it is marked as completed and crossed out.
Tasks that have already been completed can still be viewed under the tabs "All" and "Archive“ can be viewed and edited.