While manually creating a project on Codacy, on step 2 you will be presented with a public key that you will need to copy and add it as a deployment key in GitHub.
If the key is not properly created in GitHub, when trying to add the project, Codacy will try to validate the settings. If it fails, it will notify you of the reasons to have failed. If lacking the key on the repository, a small tutorial will be presented below as shown here:
If the project is successfully created, it will mean that the key has been used successfully to access your repository.