From SCOTUSblog,

“Based on two words separated by 16 others in … IEEPA—‘regulate’ and ‘importation’—the President asserts the independent power to impose tariffs on imports from any country, of any product, at any rate, for any amount of time,” Chief Justice John Roberts wrote. “Those words,” he continued, “cannot bear such weight.” “IEEPA,” Roberts added, “contains no reference to tariffs or duties.” Moreover, “until now no President has read IEEPA to confer such power.”

"Justice Brett Kavanaugh wrote the main dissent, which was joined by Justices Clarence Thomas and Samuel Alito."