Sir,
Section 122 (1) (ix) of CGST Act, 2017 stipulates that " Where a taxable person who is liable to be registered under this Act but fails to obtain registrationhe shall be liable to pay a penalty of ten thousand rupees."
In your case you have stated that your turnover is less than 20 lakhs. You produce the evidences to show that your turnover is less than 20 lakhs during previous financial year or not likely to exceed the limit of 20 lakhs during the current financial year and tell the department you are not liable to register since Section 22 91) of CGST Act, 2017 stipulates that " Every supplier shall be liable to be registered under this Act in the State or Union territory, other than special category States, from where he makes a taxable supply of goods or services or both, if his aggregate turnover in a financial year exceeds twenty lakh rupees:"
In view of the above you may plead that you are liable to be registered under GST Act and hence you have not obtained registration.