If this graph works for other elements, than the problem is not with this part. Check with Element.parameters node if the element has the ‘Tragendes Material’ parameter. This node should list all available parameters of an element.
This dyn file is a dynamo 1.3 file. Consider updating to 2.x, as 1.x is not really supported anymore, and the 2 versions are not truely compatible to each other. The only reason to use dynamo 1.x if you are using Revit 2016, but it’s not supported by Autodesk since 2018 March: The Revit Connection | The Dynamo Primer
Your graph seems good to me, so I guess the problem is with the Revit elements, some of them simply doesn’t contain the parameters you are looking for?
I cannot check your graph without the missing nodes, and without seeing the revit file, it seems like a specific problem in your Revit file. Also for me the Slice String node is also red, meaning it’s a missing node, is it missing for you as well? If a node is missing you cannot run your graph, data flow will stop at that node (but you already feeding it with an empty list, so this is not the root of your problem.)
Maybe can you create a screenshot, where every node and every node’s output is visible? Because your current screenshots don’t help at all, you don’t show the problematic parts, only parts which are after the problem. Showing only the “Empty List” output is not helpful.