GSTR -1A

If you have missed reporting an outward invoice in your GSTR-1 filing, you cannot add it to GSTR-1A, as GSTR-1A is primarily used for amendments based on recipient modifications in GSTR-2. Instead, the missed invoice must be included in the upcoming GSTR-1 filing for the subsequent month.

You are welcome.                 

U can add the left out invoice in1A before filling of 3b.

You can’t add the missed invoice in GSTR-1A, as it is meant for recipient modifications. Instead, you should report the missed invoice in the GSTR-1 of the following month.