You need to use Oauth, this is basic Auth.
Try:
curl -X POST "https://api.twitter.com/oauth2/token" -u "ConsumerKey:ConsumerSecret" -d "grant_type=client_credentials"
Then:
curl 'https://api.twitter.com/1.1/tweets/search/30day/devSandbox.json?query="some thing"&fromDate=201806100926&toDate=201806290926&maxResults=100' -H "Authorization: Bearer BearerTokenHere"