Problem:
This is not a valid source path / URL
I have been working on a project with a friend who set up a repository in Gitlab; and even though I was able to view all projects on it, I couldn’t really join the repository. I was using SourceTree and Gitlab.
While using SourceTree, attempting to clone a remote repository, was throwing the following error under Repository Type: This is not a valid source path / URL
Clicking on Clone will prompt the following error:

Solution:
The solution was simple, I just updated the Git Version. From SourceTree, go to Tools > Options > Git > Git Version > Update Embedded.

After updating the Git version, you should be able to clone the repository; and under Repository Type, you should see: This is a Git repository.

Other solution:
Try updating the Mercurial Version.
Skip the setup of Git & Mercurial in the wizard.
I’ve been stuck for WEEKS on this exact problem and would have continued being lost without you!! THANK YOU JUAN!