PayPal网关拒绝了请求。 您无法对此交易进行部分退款（＃10009：交易被拒绝）

I'm doing partial refund in my sandbox account. What i'm missing? If i use other credentials then it works fine.

I'm getting below

PayPal gateway has rejected request. You can not do a partial refund on this transaction (#10009: Transaction refused)

Things I know of that can cause a transaction to be held:

1. Risk Controls/Fraud Management Filters (if you turned them on in your account),
2. Transactions in a currency you don't hold (if the "Allow payments sent to me in a currency I do not hold" setting is set to "Ask Me"),
3. Transactions that are waiting for an eCheck to complete (if the setting to block eChecks is turned off),
4. Transactions that are being disputed, or
5. Transactions held by PayPal.

You can't really do much to prevent (d) or (e), but you can adjust your account settings to prevent the others.