Issue with SourceTree while cloning a GitHub repository

The exact error message is (as illustrated here):

 This is not a valid source path / URL

Possible cause:

• proxy settings (as in this thread)
• setup steps, with Git disabled (as in here)

  When SourceTree started for the first time, I skipped setting up Git & Mercurial in the wizard. Then I reran the wizard and chose to download and install the embedded packages.
  But it seems installing those didn't actually enable them - in the Tools -> Options dialogue they were both disabled!
  Enabling Mercurial (or Git in your case) allowed the clone dialogue to correctly identify the repo.

• credential issues (as in here, from my old answwer)

I was facing the same issue in Sourcetree for macOS:

This is not a valid source path / URL

The following solution worked for me:

1. Sourcetree > Preferences > Advanced
2. Remove the Host name
3. Clone the project again in Sourcetree
4. A prompt will pop up; enter your git credentials.

That's it, it resolved my issue.

Lastly on Mac I went to

Sourcetree->Preferences->Git->Git version->Use System Git

and it works... puf!