Starting with version 2.0.3 of Replay Music for Mac, support for Mac OS 10.10 Yosemite has been discontinued. We may be able to add Yosemite support in the coming months, but that is yet to be confirmed.
We recommend updating your Mac OS to at least 10.11. If that is not an acceptable option, you can downgrade Replay Music to version 2.0.1 which does include support for Yosemite. Download version 2.0.1 here.