The command "nuget install NUnit.Console -Version 3.9.0 -OutputDirectory testrunner" failed and exited with 127 during

Hello,
I was creating my travis.yml basing on the documentation to add nunit to the travis.yml:

Unfortunately error The command "nuget install NUnit.Console -Version 3.9.0 -OutputDirectory testrunner" failed and exited with 127 during . has occurred.

Can anyone know what might be the reason and how to fix it?

Job log: https://travis-ci.com/github/fszymaniak/restful-booker-api-tests/jobs/367670992
Raw log: https://api.travis-ci.com/v3/job/367670992/log.txt
Config: https://travis-ci.com/github/fszymaniak/restful-booker-api-tests/jobs/367670992/config

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