HELP! iFlash Gen6 does not recognize Mac Library (or vice versa)

Hi, I have a Gen6 modded with an iFlash solo & a 128 GB SD card. I sync to my Mac-based library, which is managed via the Appe Music app.

Every time I sync, the Mac does not recognize the library and tells me I can only syn the device with one computer at a time.

Any ideas on how to solve this issue? Which of the two devices stores information about previous syncs?

Thanks!