Unfortunately this limitation is expected. Currently, master account is not able to execute cards refund on behalf of their sub accounts (regardless OWNED / MANAGED) via dashboard. 

Please be informed that you can only execute refund on behalf of your master account via API here https://developers.xendit.co/api-reference/?_ga=2.80798943.970811849.1641276794-522955057.1641276794#create-refund 

Note: please don't forget to add the sub account ID which you want to execute the refund on their behalf in the "for-user-id" parameter in the header request.