Annual Return GST

I have sole proprietor . filing the monthly return , for the years 2018-2019 , filed nil return for all month.

Now trying to filing Annual Return, When filing return , it is showing error.

click here to see the error the complete Liabilities

Question

1 - If I have monthly nil return for 2018-2019 , i need to file annual return ?

2.- If  I have to file annual return - why this error ?

Error message in the attachment.

Please help to me