Hi Ömer,
I encountered the same problem. If I am not mistaken it is not possible to complete a usertask by the system, since it is not a user (even if you try to add it to the target users). You could use an executeMicroflowAsUser Java action, but I would say handling this within the workflow itself is a better solution here.
In the workflow you can use an interruptive boundary event combined with a jump to action. That way, you can continue your workflow after a set time at a specific spot. The jump to action can only be added to the interruptive boundary event if you remove the 'End' event first. Then you should be able to add the jump to action and end up with something like this.
Hope this helps!
You do not need a user to complete a task.
Update your workflow to add an expired outcome for the task. Create a scheduled event microflow and use the 'Complete user task' action to complete the task from this microflow. You can do so with the outcome to go on the expired path.