Hey @krycho515, thanks for reaching out to the Community!
In order to be able to accept the invite, every member should have the same address and country. He can check here if he has added the same country as you. He can follow these steps, if he needs to update it.
Then you can invite him again to the Premium Family plan. After he has accepted the invite, he can enjoy the Premium features.
If he gets an error message, you can attach a screenshot and I’ll take a look.
Keep me posted!