How do I retrieve my TestFlight invitation code?

The app developers have Test Flight codes, not Apple. You’ll need to request a code from the developer of the app.

You need to contact the developer of the app. No one here has Test Flight codes.

I think you're talking to a brick wall here.

The code is only redeemable for the specific app the developer issued the code for. You redeem the code in the Apple Test Flight app.