Forget to report invoice in gstr 1

As per provision of GST law, you can rectify your any mistake / omissions relating to inward Supplies (ITC) or outward Supplies in next year before due date of filing return of September,next year or due date of filing Annual return of that year, whichever is earlier.

So, as per above provision, You can show your invoices of F.Y 2019-20 not shown/missed to show in F.Y 2019-20, in GSTR 1 or in GSTR 3b in any month up to September, 2020 before due date of filing return of September, 2020 (i.e 20th October, 2020) and pay tax thereon along with interest.

Okkkk Thanks
but I have paid tax while submitting 3b and only mistake was that it was forgotten to report in gstr 1.

Then that's best, you are not required to pay any interest as you already shown that in GSTR 3b.
So you can simply show that missed invoices in current GSTR 1 return.