When users are unable to setup Fedex account in Sandbox and receives the error 'This account number was not recognized by FedEx. Please re-enter your account number, or contact FedEx to open a new account', consider the following:
1. Only test account numbers can be used when setting up Fedex account in Sandbox instance.
2. Contact FedEx and ask for a Test Account Number. Note that the use of a Live FedEx Account still results to an error when used in Sandbox.