003403: Cannot delete the default project toolbox.

Description

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

Solution

To use a different toolbox, add the toolbox to the project—it then is included 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 the Make Default option.

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