arm_validate_field_value_before_form_submission

add_filter( 'arm_validate_field_value_before_form_submission', 'your_function', 10, 3);
function your_function($return, $form, $posted_data) {
    //$return True or error message array if any.
    //$form Form Object.
    //$posted_data Posted form details.
    if($posted_data['some_key'] == '') {
        if(!is_array($return)) { $return = array(); }
        $return['some_key'] = 'Error message goes here';
    }
    return $return;
}