As there are several different fed Ex modules, we would need to know what module you are using and any specifics with your ordering process. Does it happen with any type of order that selects Fed Ex? What is the exact error message that you receive? These will all help with troubleshooting the issue.
I am in open Cart Version 126.96.36.199 and the message is upon check out when asked to select delivery method or if you ask for a quote and its just says error
Iget this message but I dont think it has anything to do with Fed ex
2013-05-07 16:06:49 - PHP Notice: Undefined index: banner_id in /home/nthole5/public_html/products/catalog/controller/module/carousel.php on line 22