If you see this message when you click 'delete' pipe you need to remove or edit the projects that are connected to the pipe.
Navigate to Projects in the left menu
Filter on the specific pipeline
Click on the three dots
4. There you can choose to remove or edit the project, if you choose edit you can easily change the pipeline if you scroll down the project card.
Now when you have moved/edited the connected project you can go back to the administration view and choose to delete the pipe.