Try to play music and see "Device Is Already Associated With an Apple ID"... but that's not possible

I recently set up iTunes match on my MacBook Pro (which is signed in with my one and only Apple ID).

But whenever I try to play music that's on my iPhone through iTunes Match, I see the following error message:

"This Device Is Already Associated With an Apple ID"

I only have one AppleID, and I've only ever had one AppleID. Both the iPhone and the MacBook Pro were purchased new (so there isn't a previous owner's phantom AppleID logged in somehow). Also, when I go into My Account in the Music App on my MacBook Pro, it shows my iPhone as being one of the devices on the same account. When I look at My Account on my iPhone, it shows the devices as including my MacBook Pro.

I don't understand why it thinks they are two different AppleIDs.

Finally, if I click "Transfer" the pop-up warning closes, but then nothing happens.