The workset can’t be deleted in the following cases:
The input worksetId is not valid in the document.
The input worksetId doesn’t represent a user-created workset, which means this workset might be a system workset.
The workset is not editable by the current user. In order to resolve this problem, please checkout the workset by calling [!:Autodesk::Revit::DB::WorksharingUtils::CheckoutWorksets].
The workset or some elements under the workset are owned by the other users. In order to resolve this problem, please ask the other users to call [!:Autodesk::Revit::DB::WorksharingUtils::RelinquishOwnership] to relinquish their ownership on the workset and elements.