Missing Sales of 2019-20

Do you mean they are omitted in GSTR 3B? or something else?

Yes, But both Gstr1 and gstr3b filed

You can considered that sale Invoice in current month GSTR 3B and GSTR 1.
But it can be done on or before Due date of September, 2020 return (i.e 20th October, 2020)

You can report that sale at the time of filing this month's GSTR-3B and also GSTR-1 along with the interest component from 20th/24th April (as the case may be) till the date of filing return, in which the sale will be reported. The last date to report any sale of FY 2019-20 shall be GSTR-3B of the month of September'2020.(i.e by 20th October)

 

 

 

You can report any transaction pertaining to FY 2019-20 in FY 2020-21 returns only upto September 2020 return.
Due date for filling GSTR-1 and GSTR-3B for September 2020 will be 11th October and 20th October respectively. So ensure that you report the transactions pertaining to previous financial year before this due dates.
Also After September 2020 returns you can not be able to carry out any amendment in the transactions reported in FY 2019-20.