Hi @sdnd2000
It could be that the 'External Ref Number' field is used in 'field mapping' under this existing relationship as shown in the dependency. I'd suggest you check it using classic UI.
In Classic UI, go to this relationship (model driven app settings -> Advanced Settings -> Customizations -> Customize this System-> Entities-> Case -> 1:N Relationships -> incident_parent_incident -> double-click to open) and, in the pop-up window that comes, click on 'Mappings' in the left side navigation pane. On the right side, you'd see the fields that are mapped. I suspect this field of yours will be visible there. Select it and press the X delete button to remove it from mapping. Save. Publish Customisation. Refresh. Try deleting the field now.

I hope this information helps.
Kind regards
Gulshan
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