I have set up a Ubuntu server which runs an cubernetes cluster. Now i want to conntect my kubernetes cluster to my GitLab repo.

But following error appears after adding the cluster to GitLab:

There was a problem authenticating with your cluster. Please ensure your CA Certificate and Token are valid.

Ubntu Kubernetes setup:

• transport-https
• docker
• kubelet, kubeadm, kubectl, kubernetes-cni

Gitlab integration:

• followed all the stepts in this tutorial: https://gitlab.com/help/user/project/clusters/add_remove_clusters.md#add-existing-cluster
• created an gitlab-admin serviceaccount
• RBAC enabled
• GitLab-Managed enabled
• API URL is correct
• certificate is formated correctly

What am I missing here?

Is there anything i have to install on my Ubunto Kubernetes cluster to be able to connect it to GitLab?