Use this CodeLet to remove "N/A" option from radio button list options. 
Place this CodeLet to your custom module file(.module) or Create a Custom module for Drupal 8 as per documentation https://www.drupal.org/docs/8/creating-custom-modules 
 

CodeLet
<?php
/**
 * Implementation of hook_element_info_alter
 * Remove N/A option from radio button field
 * @param array $types
 */
function MYMODULE_element_info_alter(array &$types) {
    if (isset(
$types['radios'])) {
       
$types['radios']['#process'][] = '_MYMODULE_remove_radio_na';
    }
}

/**
 * remove_radio_na
 * A Callback function for MYMODULE_element_info_alter
 * @param array $element
 * @return modified $element
 */
function _MYMODULE_remove_radio_na($element) {
    if (!empty(
$element['#field_name']) && $element['#field_name'] == 'field_range') {
        unset(
$element['_none']);
    }
           
    return
$element;
}
?>
Compatibility
Submitted by saru1683 on