When processing orders you might come across the "6003 The COMMERCIAL_INVOICE requires an electronic SIGNATURE" error. 

This error is shown when the ETD (Electronic Trade Documents) usage has not been enabled for your account.

NOTE: This error usually is shown together with error "7037 Harmonized code is missing or invalid for commodity #; estimated duties and taxes were not returned". To solve both issues, please note the solution for the 7037 error in this article.

To fix this issue you need to complete FedEx's requirements to use ETDs with your account as described here: https://www.fedex.com/en-us/electronic-trade-documents/enable.html 

You cannot activate electronic trade documents without uploading the letterhead and signature file to Fedex ship manager. 

Once done and approved by FedEx, this error message will not be displayed and the orders will be able to process correctly.

NOTE: An alternative to process a stuck batch is to temporarily disable transmitting ETDs, unchecking the option as mentioned in point 6 of the following article: How to set up Customs, Duties and Export information for FedEx [Step-by-Step guide]