How to get refund of ITC credit of GST if there is no output Liability of GST?

you can't get refund of GST credit..
input is refundable only either for zero rated supply or inverted duty structure..

A registered person may claim a refund of unutilized ITC on account of Inverted Duty Structure at the end of any tax period where the credit has accumulated on account of rate of tax on inputs being higher than the rate of tax on output supplies.

Exceptions where refund of unutilized input tax credit shall not be allowed in these cases

Output supplies are nil rated or fully exempt supplies except supplies of goods or services or both as may be notified by the Government on the recommendations of the Council.

If the goods exported out of India are subject to export duty.

If supplier claims refund of output tax paid under IGST Act.

If the supplier avails duty drawback or refund of IGST on such supplies.

you cannot claim itc or refund for excempted or nil rated goods