What app have you created? Not sure what you have done. Can you be a bit more specific? Any entries in any logs of relevance? What is your setup? Why haven't you been able to add the Tesla key?
Apply the latest update released a few hours ago for HomeAssistant. Bring your system down and power off and then restart. There may have been fixes that are relevant.
Have you followed the steps in the second paragraph here?
or is it the first paragraph here?
Are you sure you need the Fleet app (vehicles) and not the PowerWall (batteries) app?