Issue: You get an error while deleting a lead as mentioned in the below screenshot.

Root Cause: In this case, as shown in the below screenshot, this lead was linked with another lead existing in the system via Lead Referral Activity.

Solution: Make sure to check the activity history of the lead and check the lead referral activity or any such activity where the lead is linked with another lead. Hence, the activity has to be deleted to de-link the leads and then further proceed with the delete action as usual.