Penalty for not HSN summary in annual return

One of client is having turnover is 5 cr+. He has almost more than 3000+ purchase invoices & 5000+ sales invoice. Now he has not maintained accounts based on hsn summary wise or inventory wise.

Now its not possible to shown HSN summary of inward supplies or outward supplies in GST annual return GSTR-9.

What is the penalty if don’t show the HSN summary in annual return GSTR-9?