A user is not able to delete a field, with the error message indicating that it is used in a report. However, there is no report with the mentioned name in the error message. Examples showing the prompt and the error which follows when clicking on Continue:
Error: You cannot delete "[field label]" for the following reasons: This field is used in the following report(s): [Report View name]
The error message can also refer to report views.
The user would need to first delete the column or filter referencing the field to be deleted in the Report View mentioned in the error by going to Admin -> Configuration -> Report Views and finding the Report View that is using the field.
Once the reference is cleared, the user should be able to delete the field from the Document Type configuration.
Vault Help Documentation: N/A