This error occurred because user used wrong separator(for eg:-\) in bill number, Follow the bellow steps for resolve that error:-

Go to Final Reports > Internal Audit > Internal Audit - GSTR Mismatch > Transaction mismatch window will be appear > Feed date range > Then press enter key >  All bills shown of selected date > Press enter on selected bill > Bill details will be open > Click on Modify > Press enter & when cursor stop at the end of the item press left arrow key > Cursor move on bill number > User can remove/change the separator from the bill number or remove the dot(.) if using with the bill number > Then press enter & type SURE for change bill number > Then press Ctrl+w for Save.