best response confirmed by Wyn Hopkins (MVP)
Solution

We still have an active bug on this issue, so it's not fully resolved, but we have identified that this problem is due to interactions with the device driver for a pen.  In the Device Manager, if you disable "Human Interface Devices / HID-compliant pen" and then restart Excel, I think you'll find that you can use both the mouse and touch to create relationships using drag-and-drop.

 

Hope this helps,

-Howie [Excel BI Program Manager]