A common Automator error is to just get items, forgetting to remove an action added for testing, or that an application or service can already have items passed to the workflow. This can result in extra or duplicate input items - for example a Filter Finder Items action will apply the filters to the current workflow items, while a Find Finder Items will apply the filters to items from the specified source, adding them to the workflow items.
Your error is most likely from doubling up some of the file references. The application will already get items passed to it automatically (the items dropped onto it), and those are being added to the items returned from the Find Finder Items action, resulting in the workflow trying to trash items that have already been moved to the trash.
Automator doesn't have any control or conditional actions, so if you are running an application without the intention of dropping items onto it, each action has an
Ignore Input option that will prevent the usage of any items passed to it. By using this option on the initial Find Finder Items action, you won't have to worry about anything extra being added from dropping items onto the application.
If what you are wanting to do is make sure you haven't accidentally grouped something in with the items you are wanting to trash, using the Filter Finder Items action instead of Find Finder Items will perform the filters on the items passed to the workflow without getting any others.