003403: Cannot delete the default project toolbox.


The default project toolbox is an integral part of an ArcGIS Pro project and cannot be deleted regardless of whether or not it is used.


To use a different toolbox, add the toolbox to the project—it then shows in the Toolboxes folder—or access it from its folder location. To make a toolbox the default project toolbox, right-click the toolbox and select Make Default.

If you only want to delete the contents of the default project toolbox, you can delete them individually.