How can I clone a private GitLab repository?

You have your ssh clone statement wrong: git clone username [email protected]:root/test.git

That statement would try to clone a repository named username into the location relative to your current path, [email protected]:root/test.git.

You want to leave out username:

git clone [email protected]:root/test.git

It looks like there's not a straightforward solution for HTTPS-based cloning regarding GitLab. Therefore if you want a SSH-based cloning, you should take account these three forthcoming steps:

• Create properly an SSH key using your email used to sign up. I would use the default filename to key for Windows. Don't forget to introduce a password! (tip: you can skip this step if you already have one ssh key here)

   $ ssh-keygen -t rsa -C "[email protected]" -b 4096
  
   Generating public/private rsa key pair.
   Enter file in which to save the key ($PWD/.ssh/id_rsa): [\n]
   Enter passphrase (empty for no passphrase):[your password]
   Enter same passphrase again: [your password]
   Your identification has been saved in $PWD/.ssh/id_rsa.
   Your public key has been saved in $PWD/.ssh/id_rsa.pub.
  

• Copy and paste all content from the recently id_rsa.pub generated into Setting>SSH keys>Key from your GitLab profile.

   # Copy to clipboard
   pbcopy < ~/.ssh/id_rsa.pub
  

• Get locally connected:

   $ ssh -i $PWD/.ssh/id_rsa [email protected]
  
   Enter passphrase for key "$PWD/.ssh/id_rsa": [your password]
   PTY allocation request failed on channel 0
   Welcome to GitLab, you!
   Connection to gitlab.com closed.
  

Finally, clone any private or internal GitLab repository!

$ git clone https://git.metabarcoding.org/obitools/ROBIBarcodes.git

Cloning into 'ROBIBarcodes'...
remote: Counting objects: 69, done.
remote: Compressing objects: 100% (65/65), done.
remote: Total 69 (delta 14), reused 0 (delta 0)
Unpacking objects: 100% (69/69), done.