None of the above methods worked for my situation. I used this script to delete longhorn-system’s namespace by calling the k8s native interface.（Clusters managed by anrancher）
#!/bin/bash
NAMESPACE=$1
K8S_SERVER=https://localk8s.com/k8s/clusters/c-qftg9
TOKEN=kubeconfig-user-kxhls.c-qftg9:xdhrv62fcvzfrdv9qlr8w674q2mt77lzjts62xn7ch58dvb7qc5xdb
kubectl get namespace $NAMESPACE -o json | jq '.spec = {"finalizers":[]}' >tmp.json
curl -X PUT --data-binary @tmp.json $K8S_SERVER/api/v1/namespaces/$NAMESPACE/finalize -H "Content-Type: application/json" --header "Authorization: Bearer $TOKEN" --insecure
rm tmp.json