I’m not able to complete the course 'cause I skipped the last task. The popup message doesn’t say anything about skipping task and that it will affect the course completion. Now, I’m not able to redo or mark the task as completed so I can access another level of the course
Maybe I can help you if I’m understand your query correctly.
In the header of the webpage of your course (The top of the page), in the center there should be a botton saying mark complete. If the system doesn’t recognize your task as completed you can click there.