"This computer is already associated with an Apple ID" despite using the same ID? [iTunes Match]

I am trying to set up iTunes Match on my MacBook. I have always had the same Apple ID, for the purposes of this post I will call it hello@gmail.com. In System Preferences > Apple ID, I double checked the device's Apple ID is indeed hello@gmail.com. When I sign into iTunes, this is also the ID which gets prompted. I enter my password so that I can purchase the iTunes Match feature, and the following message appears:

This computer is already associated with an Apple ID.

If you subscribe to iTunes Match with your Apple ID, you cannot use iTunes Match, auto-download or download past purchases with a different Apple ID for 90 days.

Cancel or Transfer

I do not understand why I am getting this message, and am afraid that if I hit Transfer, I won't be able to download / access my music from my iPhone, which is precisely why I want iTunes Match in the first place.

Can anyone tell me why this error is occurring, and if hitting Transfer will prevent me from accessing that music for 90 days?

Any help would be lovely/