In order for someone to help you with API Request questions, it is critical that you post here the API Request you are using.
Remember to replace your Key with THE_KEY , or something like that.

If you are making an API Request of this type,
https://api.themoviedb.org/3/movie/550?api_key=THE_KEY&language=en-US&append_to_response=credits
then you can see the list of people at the bottom of the page.
For each person there is something like this:

0   
cast_id 4
character   "The Narrator"
credit_id   "52fe4250c3a36847f80149f3"
gender  2
id  819
name    "Edward Norton"
order   0
profile_path    "/5XBzD5WuTyVQZeS4VI25z2moMeY.jpg"

See that the item ID exists: id 819

EDIT
And see the link to that ID:
https://www.themoviedb.org/person/819

https://www.themoviedb.org/person/819-edward-norton

Oh ok,thank you so much