your repository must have a file called .gitlab-ci.yml which defines a pipeline. This is a set of tasks that are run by GitLab every time you make a change and push that change to GitLab.
The contents of .gitlab-ci.yml define exactly what commands are run, and under what circumstances.
To find out what caused your pipeline failure, click on the link to build that you can see in the screenshot, and that will take you to a log of the build job.
My guess is that you might be missing some variables such as an API token that allows you to connect to Digital Ocean, or that your .gitlab-ci.yml contains a bug somewhere.