Hello Tianxing,
Know more about configuring forwarding profiles on https://www.ibm.com/docs/en/qsip/7.4?topic=systems-configuring-forwarding-profiles
To resolve your issue, Please follow below steps.
Go to Admin -> Forwarding Destinations -> Profile Manager -> Select Default Profile -> Edit -> Search Domains under filter -> Uncheck or remove the default value in front of it -> Save -> Close Profile Manager.
Then try again to delete the property.
------------------------------
Vishal Tangadkar
IBM INDIA PVT LTD
------------------------------