
Hi, thank you for reaching out!
That message means the “Reuse Slides” feature has been officially removed (deprecated) in some versions of PowerPoint, especially in Microsoft 365 or PowerPoint for the web. Microsoft no longer supports it even if the option still appears.
Workaround:
Open both presentations > go to the thumbnail pane on the left > drag and drop the slides you want to reuse. Or you can copy and paste the slides as another reliable method.
Warm regards,
Arlene D.