WoW Refer a Friend doesn't work

I am new playing WoW, and I asked my friend to play with me.

Both of us had Battlenet account for years, and have been friends for years. He hasn't played WoW before.

He has a WoW account and has a character (level 2), never had a subscription before.

When I sent the link to my friend, it doesn't work when he logs in.

He is now waiting for this issue to be resolved before he pays for the subscription.

Does he have to pay for the subscription and then use the referral?

Any idea?