Travis & Github Deploy Key: repository not known to



I’m running a Swift project at Travis and I’m currently installing Fastlane to it. Currently, I have my certificates and other files used by Fastlane stored into another Github private repository, and I need to give Travis access to this repository. I’m currently following this tutorial here about how to setup Deploy Keys, which I guess is what I’m looking for:

But when I try to run travis encrypt-file 'github_deploy_key' It returns me repository not known to myaccount/provisions-certificates/

I’ve followed several other issues on Github, such this, this, this, this, this and finally this, but all of them are similar such as using --pro or renaming repository at .git/config, which is not my case.

I would like to know what am I doing wrong and if Deploy Keys is what I’m looking for to make Travis access outside directories.

Thanks in advance.