Hey @marcel,
Welcome to the SU Community Forum. I think the issues you are running into based on your comments in this and the other two threads you initiated are occurring because you have not yet familiarized yourself with how SketchUp works. It really is one of the easiest modeling software programs to learn on the market today, but as with any new tool, some effort and a bit of time and patience are required to master using it.
Several users have given you some good information already, but here are some additional tidbits you may find helpful in general, including the following boldfaced links to the introductory video tutorials. Viewing these in order is a good way to become acclimated to working with SU. Because you are new here, the Forum Guidelines are linked also for your convenience.
Getting Started with SketchUp - Part 1
Getting Started with SketchUp - Part 2
Getting Started with SketchUp - Part 3
Getting started with SketchUp - Part 4
I hope your participation in the Forum proves to be enjoyable and informative.