add_inv_create_items.php is the name of the file, not an include. Here is a list of the includes in the order they appear at the top of add_inv_create_items.php.
<?php require_once('Connections/firthweb_data.php'); ?>
<?php require_once( "WA_SecurityAssist/Helper_PHP.php" ); ?>
<?php require_once("WA_DataAssist/WA_AppBuilder_PHP.php"); ?>
<?php require_once('function_numbers.php'); ?>