Welcome, @Janina
Maybe try following the tutorials that are maintained by the people who write and maintain the code base instead?
If this what you were already following, you can explain more about any customization you’ve done already. Plus, the release you are working with and related details. Example: what is in the server logs for the error?
Then, to back up a bit, we had another recent question about “tools not installing from the ToolShed” recently. That issue had to do with the environment that Galaxy was originally installed into.
Let’s start there, thanks! 