Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires guzzlehttp/guzzle dev-master, found guzzlehttp/guzzle[dev-improve-checks-for-http-2, dev-test, v1.0.3, v1.0.4, v2.0.0, ..., v2.8.8, v3.0.0, ..., v3.8.1, 4.0.0-rc.1, ..., 4.x-dev, 5.0.0, ..., 5.3.x-dev, 6.0.0, ..., 6.5.x-dev, 7.0.0-beta.1, ..., 7.9.x-dev] but it does not match the constraint.