RE: Unable to delete event registration Record
Hi Fariha,
Please open browser console and switch to Network tab, clear network activities to make us could see the submission activity that we want clearly.
(But activities will generate constantly, just keep some records would be acceptable.)

then select record which you can't delete, then you will see a failed submission activity, db-click it to see error detail.

We can see detailed error message in Preview tab.

In my environment, due to the selected event registration record has been associate with a check-in record, so it can't be deleted.

After I delete associated check-in record for issued event registration record, then the record can be deleted successfully.
So you could check whether there is any other record which has been associated with your event registration records.
Regards,
Clofly