Why can I not merge content from an Active Presentation in Modern Authoring?

If you cannot merge content from an active presentation, and you are the Author or a Company Administrator for the presentation, check to be sure that the presentation is set to allow copying. 

To check this setting follow the steps below:

1. Edit Presentation
2. Navigate to the Settings tab in the top right. 
3. Select Access on the side navigation.
4. Confirm under the Permissions Section, that Allow authors to copy presentation is selected. 

Presentations must be set to allow copying if they are to be merged.