You must be eligible for input credit tax before you can claim it. It is important to note that you can avail the input credit only if the supplier has deposited the tax which has been collected from you during the transaction that took place between you two. That is, the verification process of the input credit is mandatory before you can claim ITC. Also, the suppliers have to be GST compliant in order for you to claim input credit.
You can claim a refund when the tax on the sale is lesser than the tax on the purchase. In such cases, there is often unclaimed input credit. In this case, you have an additional option of carrying it forward. You can claim the input tax credit on capital goods. However, you cannot claim input credit on all purchases. For example, input tax cannot be availed when you buy goods and services for your personal use. You cannot claim input credit if the purchase invoice is older than one year in most cases.