GSTR-3B tab will not open before the filing of GSTR-1
As per update from GSTN a taxpayer cannot file Form GSTR-3B return until Form GSTR-1 return has been filed for a particular period.
Now on GST portal if you tried to file GSTR-3B before filing GSTR-1 of a particular period, a message get populated there saying;
"You have not filed GSTR-1 of the selected tax period. Please file the same to enable filing of GSTR-3B"