Good day @luke__nguyen,
Thank you for posting to the PayPal community.
Please make sure the currency setup in your PayPal Live/Sandbox seller account and also check your Payment Receiving Preferences settings to allow payments that are sent to you in a currency you don't hold.
Also, the transactions your trying to refund should be in "completed" status.
Once the above steps is done, please try processing a refund and make sure your passing correct amount value.
If your still facing any issues, please create a MTS ticket via - https://www.paypal-support.com/s/?language=en_US with the detail information and error details.
If this post or any other was helpful, please enrich the community by giving kudos or accepting it as a solution.