Typescript React <Select> onChange handler type error

Since MUI Select in not a real select element you will need to cast e.target.value using as Type and type the handler as React.ChangeEvent<{ value: unknown }>

const handleCountryChange = (event: React.ChangeEvent<{ value: unknown }>) => {
  setValues({...values, country: event.target.value as string});
};

Seemed counterintuitive for me either, but I had to do this:

const handleChange = (e: ChangeEvent<{ value: string | unknown }>) => formik.setFieldValue('field', e.target.value)

This is proper for the latest MUI 5 / Select

  import { SelectChangeEvent } from "@mui/material";

   const handleOnChange = (event: SelectChangeEvent<unknown>) => {
    const value = event.target.value as YourEnumType;
  };