How can I enable return/refund for the users on my store?

To enable return and refunds on your store, go to Settings > Miscellaneous settings in your admin panel. Select the checkbox to enable the order returns and refunds.

For more information, please refer:
https://help.storehippo.com/topic/miscellaneous-settings

To know more about refunds and returns, please refer the following links:
https://help.storehippo.com/topic/returning-an-order
https://help.storehippo.com/topic/refunding-an-order