One possible reason you can’t delete the purchase order is that there might be a custom field with the List/Record or Multi-Select field type that needs to be reconfigured. This custom field may have been applied to other transaction forms and is referencing the purchase order, which is why you are encountering the error message.
To verify this, you can try the following steps:
1. Navigate to Personalization > Lists, records, and fields > Transaction body fields
2. Click Edit next to the Custom Field applied to other transactions with List/Record or Multiple Selection field type
3. In the Validation and Standardization tab, look for the field “Allow deletion of list/record values?”
4. Set the value to “Allow and set dependent field values to null”
5. Click Save
For more information about this, see the SuiteAnswers article ID 36717 Customizing Delete Behavior for Records Referenced by Custom Fields.