When going to Europe, travelers are required to hold a valid visa. Currently nationals from 60 countries, including U.S. Citizens, can apply for the ETIAS Authorization for stays of up to 90 days within a 180 day period. However, if you plan to stay in the Schengen Area longer than 90 days or are not eligible for the ETIAS visa waiver, you will need to apply for a different EU visa, like the Schengen Visa.
If you already have a Schengen Visa you do not need to apply for ETIAS. You can travel to the EU with your valid Schengen Visa. However, if your Schengen Visa has expired you will have to apply for an ETIAS travel authorization.