This error indicates
git clone fails due to missing username and password. Drone provides username and password for private repositories. The most common reasons one encounters this error are the following:
Gitlab is running in private mode
Private mode means public repositories still require username and password to clone. If you have this Gitlab configuration, you need to include
DRONE_GITLAB_PRIVATE_MODE=true in your server configuration. This instructs Drone to treat public repositories as private, for authentication purposes. Note you will need to delete / re-add your repository for this to work
Repository changed to private
Sometimes people add a public repository to Drone, but then change the repository to private in Gitlab. The result is that Drone attempts to clone the repository without username and password, and it fails because the repository is now private. To resolve this issue, you need to remove and re-add the repository so it is correctly labeled as private.