In order to print shipping labels you will need to follow the instructions here to certify your API connect with FedEx. https://developer.fedex.com/api/en-us/certification.html
Steps 1-2 should already be completed if you completed the FedEx API setup here
Download and complete the PDF form on step #3 of the certification process on the FedEx website.
Complete the remaining steps in the certification process with FedEx.