FedEx has separate APIs for test and production labels. You'll have 2 unique sets of account credentials with FedEx: one production and one test. Login here for your test credentials (screenshot below):
You'll be able to access both of the FedEx APIs via Shippo but you'll need to register both accounts within Shippo, either via our dashboard or API endpoint. After registering both FedEx accounts you'll leave test toggled off for the production FedEx account and test toggled on for the test account (screenshot below).
When creating production FedEx labels via API you'll use your Live Shippo API token. For the test FedEx account you'll use your Test Shippo API token.
Note: A mismatch of your live/test token with the live/test FedEx account will return a 403 error from Shippo.