See
For an example. You are not sending a header correctly, and can’t tell you how to fix that since you didn’t provide what language you are using or sample code.
See also
curl -H 'Client-ID: uo6dggojyb8d6soh92zknwmi5ej1q2' \
-X GET 'https://api.twitch.tv/helix/streams?first=20'