The minimum editor version for Ultimate Game Music on the Unity Asset Store is changing, from 5.1.1 to 2017.4.
If you still wish to use Ultimate Game Music in projects older than 2017.4 then you still can, however you may require Unity 2017.4 or newer to download the asset or to receive future updates and new content from the Asset Store.
To transfer Ultimate Game Music to an older Unity project, simply import the asset into a 2017.4 project or newer and copy the files you wish to use from the project folder. Ultimate Game Music is made up of WAV audio files, compatible with any version of Unity.
If you are already using Ultimate Game Music and have the most recent 1.8 update (March 2018), then there’s no need to download this new version. This is an administrative change and no new content is included.
A content update (v1.9) is planned for later this year and will be free to all Ultimate Game Music customers.