It looks like Google is no longer letting users to gift Play Music subscriptions via Android. As many you may already know, Google is moving away from Play Music to YouTube Music being their music service of choice moving forward.
Right now, if you trying to gift a Play Music subscription the following alert will come up on Android:
“Important – Google Play Music subscriptions can no longer be purchased as gifts.”
Now, if you still want to gift someone a Play music subscription, either use an older version of the Android app or buy it via the web app. You can also just gift a Google Music subscription, which still let’s you use the Google Play Music app.