Thank you for your question.
I’ve just tried to replicate your issue using the project files from the tutorial and updating to the latest version of Unity. Connecting up the RigidbodyFirstPersonController seems to be working fine for me.
The fields you have added to the Settings Controller look correct, and the field has appeared in the Inspector as you would expect… So that rules out a few potential problems.
One thing to try:
If you click on the little connection circle at the end of the inspector field what do you see?
Making sure scene is selected. Can you see and manually select RigidbodyFPSController?
Please let me know if you are seeing something different or whether manually selecting in this window does not complete the inspector field.
I do occasionally find that Unity misbehaves in the strangest ways, sometimes it just needs to be closed and restarted, or updated to fix the odd little bug.
let me know how you get on.