As Google is decommissioning their Google Play Music service you have to transfer to YouTubeMusic
Upon selecting the Transfer link I got
YouTube channel you’re currently using isn’t supported for the Google Play Music transfer.
This is because my Youtube was a brand account
You can move your Brand account to a Google account so that all your music history data is there: https://support.google.com/youtube/answer/3056283?hl=en
if you go to your advanced account settings: http://youtube.com/account_advanced
After this it will let you transfer
Trackback from your site.