When attempting to merge two records in The Mergician, the control report returns the error:

Error merging constituents. Reported Error is: Error performing Outlook operation. Access denied

This article on the Blackbaud knowledgebase talks to this:

https://kb.blackbaud.com/articles/Article/116420 (A program is trying to send an e-mail message on your behalf. If this is unexpected, click Deny and verify your antivirus software is up-to-date.)

This happens when Auto-Remind is marked on one of the constituent record and Notify using "Both" Raiser's Edge reminders and Outlook Appointments is marked.