Class 'GuzzleHttp\HandlerStack' not found

I'm trying to run LaunchDarkly and within the code it says that I need to initialize my class like so

$client = new LDClient("MY_CLIENT_KEY");

Now I was able to do that but an error occured below is the error

The error is within this code $stack = HandlerStack::create(); inisde the GuzzleFeatureRequester.php. Its trying to call

use GuzzleHttp\HandlerStack;

but this class does not exist when searching. Any idea on how to fix this? I'm using this on my composer.json

"guzzlehttp/guzzle": "~5.3.0",
"launchdarkly/launchdarkly-php": "^2.0"

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