@Tekilamuerte said:
I try to get API from /movie/popular but appear an error Response 503 Service Unavailable.
I already register and get my Api key, but when I try to use my ApiKey in the lists, and I receive this message
Response 503 Service Unavailable.

Error: Network Error
Check the developer tools console, it might have more information on the error.
If you are using an Ad blocker, it is possible your Ad blocker is blocking the request.

I was looking a searching in the forum but the answers is the you have to use a different VPN but also when I change VPN doesn't work
¡¡Someone can help me please!!

You may be using Try it out feature.

You are probably sending your API Request directly from the Developers page.
https://developers.themoviedb.org/3/getting-started/introduction
Developers.themoviedb.org is hosted by https://stoplight.io/
And yes, he's in trouble.
This is the error response I get on developers.themoviedb:

Response    503 Service Unavailable  

Error: Network Error  
Check the developer tools console, it might have more information on the error.  
If you are using an Ad blocker, it is possible your Ad blocker is blocking the request.  

But if I use the URL directly in the browser I get the correct answer.
Copy the URL of the API Request and paste it into a new tab or window of your browser.

Travis Bell, the Manager, said (2021-01-15):
https://www.themoviedb.org/talk/600184fc6aa8e0004077e364?page=1#6001bbacbe4b36003d514ac3

Hi @ user,
I believe Stoplight has temporarily blocked the Try it out feature for our account
because too many people were using it as the source for their apps,
and not using api.themoviedb.org for some reason.
No timeline on getting it restored, but that's the problem.
Switch to use our proper production address, api.themoviedb.org

And here (2021-06-04)
https://www.themoviedb.org/talk/60ba469218b75100785c163b#60ba6c76ffd44d0079eaa363