Hello Jürg S,
Welcome to the Microsoft Q&A forum.
When you create an Azure Synapse Analytics workspace, you need to provide a unique name for the workspace. If you try to create a workspace with a name that already exists, you will receive an error message indicating that the name is not available. The name must be a unique across the entire Azure cloud
To resolve this issue, you can try creating a new workspace with a different name.
I hope this helps. Please let me know if you have any further questions.