GST missing mar 2019 purchase bill

Yes, you can claim input on purchase invoices for the year 18-19 till the filing of GSTR3B for september 2019.

In case of discrepancies of input tax between GSTR3B and GSRT2A, if the input taken in GSTR3B is as stated in the invoice, then you have to contact the other party and ask them to amend their GSTR1. On the other hand, if the bill given in GSTR2A is correct, then you have to adjust the input amount in GSTR3B (increase or decrease) to match the amount given in GSTR2A