how can i apply the material? im new to blender so it might be really stupid what im asking, but ive finished my first little project and onlythe material for one piece is missing, i try to add a metllic material and the preview of the material looks right, but on my project it isnt working. Maybe sb could help me ß
If you look at your materials tab in the properties panel, you'll notice that there are several different material slots. The one you want to use (Material.004) is not the first one, so it is not the default one that Blender will choose.

To fix this, go into edit mode, select the faces you wish this material to affect, and click on the "assign" button in the Materials tab.