sign-in required when trying to buy apple music service

I followed the steps to buy an Apple music service but after I typed in the SMS code, the following words come out:

"Sign-In Required

If you have an Apple ID, sign in with it here. If you have used the iTunes Store or iCloud, for example, you have an Apple ID."

Those words don't even make sense.

And it asks me to type in a "password" which I totally don't know what it stands for.

Below on the left writes "cancel", "Buy" on the right.

I tried to type in my Apple Id and tap "Buy", but then it would jump to the window before the purchase. I would have to start all over again and face the same problem again.

So anyone can help me solve this problem? It would be a great help.

Thank you!