You need to set a header declaring you want v5 responses.
Without a header it defaults to v3 and you are using V3
For exmaple:
curl -H 'Accept: application/vnd.twitchtv.v5+json' \
-H 'Client-ID: uo6dggojyb8d6soh92zknwmi5ej1q2' \
-X GET https://api.twitch.tv/kraken/users?login=dallas,dallasnchains
You need an Accept header in your request declaring v5. Otherwise this end point attempts to lookup by username 262027661 instead of userid 262027661 as thats the behaviour of this end point between v3/v5