Error: "WorkflowServiceStore is not a Farm Level Feature and is not found in a Site level defined by the Url"

When I saw the classic message "SharePoint 2013 Workflow platform is not available" in SharePoint 2013 Designer, I thought it's easy to fix.

1. Confirm Workflow Manager Client is installed on all SharePoint servers.
2. Register workflow service for the farm.

3. Enable "WorkflowServiceStore" feature on the site.

However, the script "Enable-SPFeature -Identity WorkflowServiceStore -url $webUrl -Force" throw out the error message below:

Enable-SPFeature : The Feature is not a Farm Level Feature and is not found in a Site level defined by the Url $webUrl.

At line:1 char:1

+ Enable-SPFeature -Identity WorkflowServiceStore -url $webUrl ...

+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    + CategoryInfo          : InvalidData: (Microsoft.Share...etEnableFeature:

   SPCmdletEnableFeature) [Enable-SPFeature], SPCmdletException

    + FullyQualifiedErrorId : Microsoft.SharePoint.PowerShell.SPCmdletEnableFeature

After 30 minutes struggling, finally I realized that the site was still in "SharePoint 2010 mode", so the SPWeb level feature "WorkflowServiceStore" doesn't exist!

The error is quite obvious, but, I wish SharePoint 2013 Designer could provide more informative message  :-(