Does your declaration have a status of ‘X days overdue’? Submit it as soon as possible to avoid extra costs. You can read more about that here: Overdue declaration: what are the consequences?
You can submit the serial numbers of that declaration only after you have submitted the final declaration. Then you will see an overview of all batteries for which serial numbers are required under ‘Serial numbers’.
Is there a lock icon 
on your overdue declaration? This means that the allowed time limit has been exceeded and therefore you can no longer submit this declaration.