GSTR 3B FOR DEC nil return NEED payment

I have one GSTR 3B query, I have filed December month nil return, we choose nil return column yes , other option are No,
And submited, while we go we file return, it asks for first clear your liability,
Here 2 questions arises
1. How do I know my liability
2. We did return file within due date, why it ask for payment? Hints : We clear all previous month all liability.


Please clarify the gst return query