What GSTR-2B is
GSTR-2B is the input tax credit statement the GST portal generates for each period from the invoices your suppliers report. It is generated on the 14th of the following month, and it is the reference for the input tax credit you claim in GSTR-3B.
Input tax credit can generally be claimed only on invoices that appear in your GSTR-2B, so a supplier invoice that is missing from it is credit you cannot take yet.