I visited S.Korea Seoul 6 months ago… You must have a valid U.S. passport to enter Korea. From April 1, 2023, to December 31, 2024, the Korean Electronic Travel Authorization (K-ETA) is not required for US citizens for stays of 90 days or less that are for tourism or business purposes.
You didn’t mention your country which would be helpful…. easy enough to use google for your specific country..