Copy from http://cgit.drupalcode.org/tincanapi/plain/.gitlab-ci.yml

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Conclusion: remove symfony/console v2.8.28
    - Installation request for sebastian/phpcpd ^3.0 -> satisfiable by sebastian/phpcpd[3.0.0].
    - Conclusion: don't install symfony/console v3.3.10
    - Conclusion: don't install symfony/console v2.8.28|install symfony/console v3.3.10