What is the remedy available to the taxable person if the order passed u/s 64 is erroneous

On an application made in FORM GST ASMT–17 within 30 days by taxable person from the date of receipt of order passed summary assessment order the Additional/Joint Commissioner may withdraw such order and follow the procedure laid down in Section 73 or 74 which provides for determination of tax liability on account of tax not paid other than fraud, wilful mis-statement etc., or otherwise.

(ICAI FAQ PUBLICATIONS 06-09-2017 SUMMARY ASSESSMENT IN CERTAIN SPECIAL CASES: FAQ NO. 26)

What is the Time limit for passing the assessment order on the unregistered person

The proper officer, in relation to assessment of taxes on the unregistered taxable person, shall issue the assessment order in FORM GST ASMT-15 within 5 years from the date specified under Section 44 for furnishing of the annual return for the financial year to which the tax not paid relates.

(ICAI FAQ PUBLICATIONS 06-09-2017 ASSESSMENT OF UNREGISTERED PERSONS: FAQ NO. 21)

Whether, The CGST Act, 2017 provides for assessment of taxes on the unregistered taxable person who fails to take registration even though liable to do so? What will be the procedure of such assessment

If a taxable person fails to obtain registration even though liable to do so or whose registration has been cancelled under section 29(2) but who was liable to pay tax, the proper officer may assess the tax liability to the best of his judgement after providing opportunity of being heard to such person.
The proper officer shall issue a notice to a taxable person in FORM GST ASMT-14 containing the grounds on which the assessment is proposed to be made on best judgment basis.                                                                                                                     The registered person will be allowed a time of fifteen days to furnish his reply.

(ICAI FAQ PUBLICATIONS 06-09-2017 ASSESSMENT OF UNREGISTERED PERSONS : FAQ NO.  20)

Whether, The CGST Act, 2017 provides for assessment in case of taxable person who does not furnish returns under Section 39 and 45

In terms of Section 62(1) of the CGST Act, 2017, the proper officer is empowered to assess the tax liability on such registered person to the best of his judgment taking into account all the relevant materials which is available or which is gathered and issue an assessment order in FORM GST ASMT-13 within a period of five years from the date specified under Section 44 for furnishing of the annual return for the financial year to which the tax not paid relates.

(ICAI FAQ PUBLICATIONS 06-90-2107 ASSESSMENT OF NON-FILERS OF RETURNS: FAQ NO. 17)