Is HSN wise summary mandatory in GSTR 1?

NO.

Minimum digit required to be mentioned in the tax invoice and consequently to be reported is as follows:

1. Up to 1.5 crore annual turnover - Nil digit 

2. More than 1.5 crore and up to 5 crores annual turnover - 2 digit  

3. More than 5 crores annual turnover - 4 digit

Not mandatory for whose Turnover is below 1.5 CR, mandatory for turnover exceeding Rs.1.5 CR

turnover not exceeding 1.5 cr then can i skip hsn summary wise box in gstr 1>

dear Mr.Jack b,  Then, no problem.

No, it is not mandatory for a person for a person having turnover less than 1.5 crore.

If turnover exceeds 1.5cr and if we have not disclose HSN wise summary, then what are its consequences?

No it is not mandatory to upload HSN details if your turnover is less than 1.5 crore

HSN wise summary reporting is mandated in GSTR-1 and GSTR-2, with the exception that where HSN is not available, a descripttion of goods & services can be provided. 

But the turnover below 1.5 crore does not need HSN details to be included. 

HELLO FRIENDS I FILED GSTR-1 OF APRIL 2021 WITHOUT DISCLOSING HSN SUMMARY. WILL I BE PENALISED. IF YES, THEN HOW MUCH. OR IS THERE ANY WAY TO RECTIFY THE GSTR-1.