Yup it seems like both the fields have a different data type. so you will have to match those. Usually it's best practice to first import the CSV as a geodatabase table to your database using the "table to geodatabase" tool. And then change the data type accordingly.
CSVs when brought into Pro sometimes have a habit to assigning wrong data types to fields. So maybe the FIPS column in your CSV data was taken as a text field.
Usually with FIPS I just convert the column to a numeric data type as that also removes any extra spaces if there are any.
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