European phone can't register to AT&T: Error GA205

Hi,

my son arrived in the US last Thursday and he wants to activate an AT&T prepaid SIM card.

He get's a text message with error code GA205, stating in the text "the phone you recently tried to activate won't work"

He went to the store again today and when he was in the store, the shop assistant was able to call him on his US number.

But once he was back in his room, he again got the same text message.

What can be wrong?

The phone is a (european) Samsung Galaxy S8

Thanks in advance.

Regards,

Luc