Can use apple music app in my ipad and iphone with individual subscription and same Apple id?

You can only listen on one device at a time, but yes, you can be signed in to both devices with the same Apple ID. If you try to listen simultaneously, however, you won't be able to. In other words, if you are trying to use this as a way of allowing multiple people access to your library without paying for a family plan, Apple has made that impossible.

Try signing out of your iPad then sign back in and make sure sync library is turned on in settings> Music

You’re welcome