Warning, with GitLab 13.11 (April 2021):

One-click GitLab Managed Apps will be removed in GitLab 14.0

We are deprecating one-click install of GitLab Managed Apps.

Although they made it very easy to get started with deploying to Kubernetes from GitLab, the overarching community feedback was that they were not flexible or customizable enough for real-world Kubernetes applications.

Instead, our future direction will focus on installing apps on Kubernetes via GitLab CI/CD in order to provide a better balance between ease-of-use and expansive customization.

We plan to remove one-click Managed Apps completely in GitLab version 14.0.
This will not affect how existing managed applications run inside your cluster, however, you’ll no longer have the ability to modify those applications via the GitLab UI.

We recommend cluster administrators plan to migrate any existing managed applications by reinstalling them either manually or via CI/CD. Migration instructions will be available in our documentation later.

For users of alerts on managed Prometheus, in GitLab version 14.0, we will also remove the ability to setup/modify alerts from the GitLab UI. This change is necessary because the existing solution will no longer function once managed Prometheus is removed.

Deprecation date: May 22, 2021

I had was facing the same issue, In my case it was because i had not set RBAC-enabled cluster to true. I deleted the intergration and checked RBAC-enabled cluster when i re-integrated and it worked.

Runner logs:

kubectl logs install-runner -n  gitlab-managed-apps

Error: query: failed to query with labels: secrets is forbidden: User "system:serviceaccount:gitlab-managed-apps:default" cannot list resource "secrets" in API group "" in the namespace "gitlab-managed-apps"

Reference:

gitlab issue