Same problem here. Pretty odd not to support your main desktop OS at launch.
Glad to know I'm not the only one.
Interestingly the "Try Now" opens iTunes which in turn asks you to upgrade iTunes which in turn leads you to a site asking you to download the old 12.1 - Bit of a hick-up there.
hiccup.
:)
hiccup!
OS X just updated to 10.10.4, available now & free.
I just updated to 10.10.4 but it doesn't update iTunes to 12.2, the version required for Apple Music. Guess it'll be out later today.