WebResource_MultiSelect1 is not a field but a WebResource, this means you will not find inside the right list of fields.
You need to check manually the other components inside your form (by the Properties button on the ribbon when you are in the edit mode) if one is called with that name.
You can also attach a screenshot of your form if you prefer. There is a part containing several checkboxes?
Another thing: because is a WebResource sure inside your system there is an HTML Web Resource to manage this MultiSelect functionality, you can check the list of components inside the default solution.