Si conectando la cuenta nos da error de que no existe el repositorio seguramente es porque NO tenemos accesos de escritura …. bla bla bla

Searching around, I found another workaround, which finally gave me the full access:

1) Go to https://github.com/settings/tokens
2) Click «Generate new token»
3) Input token description e.g. «Sourcetree Mac Token», select «repo» checkbox, and click «Generate token»
4) Copy the generated token
5) Add your GitHub account to Sourcetree, but now rather than using OAuth, select Basic authentication
6) Input your username
7) Paste the generated token as password

Now you should have a fully functional connection between your GitHub account and Sourcetree, including access to all private repos. If you encounter any functionality that is not working (I haven’t), try regenerating the token with more permissions – I intentionally selected only «repo», as that is all I need at this moment.

Hope this is helpful, until there is a fix from Sourcetree team. As mentioned above, the issue has already been filed here https://jira.atlassian.com/browse/SRCTREE-6322