If you are suddenly unable to generate an e-invoice for a previously working HSN code in BUSY,it is possible that your turnover may have increased,if your turnover is above 5 crore,so We recommend specifying the length of your HSN code as 6 to 8 digits in the BUSY,so if you are mention 4 digit HSN in BUSY,and its not working now,so you need to increase your HSN length in Item master,Press ALT + M in item feild,open it modify mode and mention 8 digit HSN,and validate,save this master then you will generate an e invoice,it will be generted.