<Errors>
<ShortMessage>PayPal may delay the settlement of funds to ensure smooth transactions.</ShortMessage>
<LongMessage>PayPal may delay the settlement of funds to ensure smooth transactions.</LongMessage>
<ErrorCode>21915465</ErrorCode>
<SeverityCode>Warning</SeverityCode>
<ErrorParameters ParamID="0">
<Value>21</Value>
</ErrorParameters>
<ErrorClassification>RequestError</ErrorClassification>
</Errors>
Summary
From a technical perspective, this is a warning, your listing will not fail because of this warning and your application does not need to take any further action.
PayPal may temporarily hold funds to cover transaction liabilities like reversals or chargebacks, which is why you may see this warning in your AddItem / RelistItem family of calls. To get more detailed information on reserves, please see the help page.