Show cause notice received for cancellation of registration

You cannot cancel your registration without furnishings your returns.
As per the provision of Section 29 of CGST Act the office can issue SCN for cancellation of Registration.

Hence in term of Rule 21A notice is issued you in Form REG 17 , so you are advised to furnish all your return till date .

Firstly you have to file returns as per section 39 after that only you can cancel your registration. if there is no transactions you have to file nil return s.now for nil return there is no late fee as per the notification .

Yes, You are required to file all your pending Returns till date before cancellation of GST registration.
If there is not business, You can file NIL return.
And file it before 30th September, 2020 because up to this date there is no late fee for NIL Return.
So, it's advisable to file all your return as earlier as possible.