Missed invoice of Feb-2019 in GSTR1.

Yes.Agree with above view, no adjustments can be made after September or before filing Annual Return which ever is earlier.

Upto march 19 invoices only can declare before 2019 september sir?

If I have september 2019 invoices can declared in october or november right sir?

yes your understanding is correct.

But I am not making any changes, I have already paid taxes in 3b but just failed to add invoice in R-1. Still I cannot add invoice now?