Fleets require Cloud Service Mesh to be installed in your cluster. Before you can install the Knative serving fleet component, you must first migrate your Istio installation to Cloud Service Mesh.
By default, the previous version of Knative serving, also referred to as the "GKE add-on", includes and installs a bundled version of Istio. There was also the option to install an alternate version of Istio, formally named Istio on GKE and referred to as the "Istio add-on".
Depending on the version of Istio that you have installed and configured to use with Knative serving, you must follow the corresponding guide to migrate to Cloud Service Mesh:
Bundled version of Istio: The bundled version of Istio is uninstalled when you performed the uninstall process of the Knative serving "GKE add-on". You can now proceed to installing and configuring Cloud Service Mesh.
Istio on GKE: If you installed the "Istio add-on", there are two possible paths for migrating to Cloud Service Mesh. To proceed, you must follow the detailed steps to identify which path you must take to migrate from the "Istio add-on".