If you see "Not all lines are completed" in the progression panel in the right-hand part of the subtitle editor, as shown on the screenshot below:



it means that at least one subtitle lacks text or timing. Subtitles that have empty or untimed sells, cannot be marked as completed.


Solution: 


1. Check all the subtitles, either in the editor or in the subtitle view. Make sure you check the end of the subtitle set - typically (but not always) incomplete subtitles can be found there:


- In the editor



- In the subtitle view


2. If you find empty cells anywhere, fill them with text or delete them:




3. If you find subtitles that lack timing, sync or delete them:



4. When everything has been corrected, your progression panel will look as on the screenshot below:



Now you should be able to make a final review of your work (if necessary) and mark your subtitles as complete.