I recently moved from the US to Spain, however, I started a family plan with PayPal linked to the united states and my son has his account set to Spain and he can't change it and now he isn't able to join my family plan.
Hey @agustindoeyo, welcome to the community!
Since you're both living in Spain and you must live in the same physical address to join the plan, you'll need to update your account's country using a payment method from Spain.
Your son will then be able to join the Premium for Family plan.
I hope this clears things out.
All the best.