The Trump administration said the first tariff refund payment is set to go out around May 11, even as thousands of US importers encounter issues with a new online portal designed to handle claims on the $166 billion in duties that were overturned by the Supreme Court.

The Trump administration has begun returning $166 billion in tariff refunds, launching a new portal for U.S. importers to claim their money back, but consumers who paid higher prices on imported goods...