Здесь установлен фиксированный код
<Controller
as={
<KeyboardDatePicker
autoOk
disableToolbar
variant="inline"
format="MM/dd/yyyy"
id={"appointmentDate"}
// inputRef={register({ required: true })} // not required remove this
inputVariant="outlined"
label={"Appointment Date"}
required={true}
helperText={errors["appointmentDate"] && "Required..!!"}
error={errors["appointmentDate"] ? true : false}
KeyboardButtonProps={{
"aria-label": "change date"
}}
/>
}
// rules={{ validate: value => value === null || "Required ..!!" }} // change this like below
rules={{ required: true }}
name="appointmentDate"
control={control}
/>
https://codesandbox.io/s/mui-autocomplete-with-react-hook-form-iymgr