I am getting the same error. I am in a timed interview code challenge and need this to start working.

@artur.yurovskikh@gmail.com said:
API key
Hi! For some reason I can't use my API key. Using the API Read access token works,
but whenever I try to use the API key I get the following error:

{
"status_code": 7,
"status_message": "Invalid API key: You must be granted a valid key.",
"success": false
}

Could you please help? Thanks!

I assume you are testing/using "Try it!" in the documentation.

The "Try it!" option in DOCs it only works with the "Access Token Auth" option.
It doesn't work if you select the "API Key Auth" option.
Your API Key validation test is done using your Token.
https://developer.themoviedb.org/reference/authentication-validate-key
If your Token works then your API Key must also work.

If you want to test your API Key directly, follow the instructions below.

Take a Test
Using a computer, desktop or notebook.
On some mobile devices this test may not work correctly.
In the URL below, change THE_KEY to your API Key.
https://api.themoviedb.org/3/movie/550?api_key=THE_KEY
Use this example to do a search using a new tab or window in your browser.
You will receive the basic data of the Movie Fight Club (1999)
https://www.themoviedb.org/movie/550-fight-club
If it works, your Key is OK.

When there is a problem, the TMDb Server sends a message.
Can you tell which message the server sent?
Here is a list of possible messages:
https://developer.themoviedb.org/docs/errors

If it does not work, let us know right here.