Missing Invoice of March Month.

You can show the Missed invoice in June returns

Brother financial year changed ?

ITC can be claimed only within one year from the date of issue of tax invoice relating to supply. In any other case, the last date to claim ITC is earlier of the two below:

Before the filing of valid return for the month of September following the end of FY to which such invoice is related, or Before the filing of the annual return

Note: As per section 44, the last date for filing of annual return is 31st December following the end of the financial year.

In GSTR3B returns, you can account the Missed invoices in succeeding month returns.

this is possible in Sales invoice ?

Sales invoice can be accounted in next gstr1 with original invoice date.