There can be several reasons for not being able to add a GIT repo to your project. One of the less common reasons for failure to add a GIT is that you may be trying to add a GIT repo whose URI is not in the form
Some GIT servers are setup to use a username that is not 'git', so the URI would look like
At the moment this is not supported by Domino and a repo that has a URI with a non 'git' user cannot be added directly to a Domino project.
To work around this you will need to access your repo using standard git push/pull/fetch/commit commands from within your execution. In a job this would be in your code and in a workspace these can be executed from the workspace terminal command line.
We are looking in to adding this functionality in a future version of Domino, but as of Domino v5.2 this is the current situation.