Hey Madhavi T,
"Referenced by" errors mean that there is something in the system that is directly referencing that asset. If you'd like to delete the asset, you first need to get rid of whatever is referencing it. You'll see the same error if you try to delete an email that's referenced by a smart campaign.
I've been able to replicated this issue in an instance and am seeing that even if you don't save the drill down, this error does occur. If you can still access the URL string for that drill down you might be able to resolve this by deleting the drill down report from that URL. You may be able to figure out what the URL string is based on the Drill Down ID provided in the error. I've tested this in my instance and found that worked.
Otherwise, might be a bug that you need to reach out to Marketo support to resolve.
To view/access the dependencies on the report to be deleted:
1. Click the report from the Reports folder.
2. Click the Smart List tab.
3. Click View Qualified Leads.
4. Within the new tab that opens, click Used By. The dependencies are listed here.
You can click the report links here and delete them by using Report Actions->Delete Report.
Once there are no dependencies, you can delete the original report successfully.