Sounds like something that Google will want to address immediately, right? However according to the YouTube Help page, it seems that even Google has no idea what is causing the lock out for their users, and it looks like there is no workaround or fix available at the moment either which safe to say is something that a paid subscriber definitely does not want to be hearing!
However Google has since updated the help page which now reads, “Some Google Play Music subscribers are currently not being given access to YouTube Music Key. Normally, Music Key access is automatically granted if you’re in a launched country, and you’re using YouTube with the same account that your Play Music subscription is on. We’re working on fixing this, please stay tuned.”
Previously the last sentence read, “Unfortunately there’s no workaround,” which didn’t exactly fill us with confidence. No word on when Google expects the issue to be resolved, but in the meantime we guess YouTube Music Key subscribers will just have to listen to and watch music videos like the rest of us.