Based on this thread :
Maybe the notebook is still open in another browser tab or window?
Restart the synapse woerkspace and then try to delete the notebook again.
You can follow these steps Open the Azure Synapse Studio and go to the workspace where the notebook is located. Click on the Notebooks tab and locate the notebook(Your Notebook) you want to delete. Right-click on the notebook and select "Delete" from the context menu. Confirm the deletion when prompted. Refresh the browser page to ensure that the notebook has been successfully deleted.