why i can't buy a new iphone ?

Apple imposed the 2 iPhones of the same model purchase limit last March to try to deal with reduced production capacity due to COVID-19. Since the global electronics industry is still dealing with shortage of supply of most parts, that limit will probably be around for awhile.

Two is the limit.

Since we are all other users and do not work for Apple, we do not have access to anyone's accounts or Apple's requirements for orders, etc. So, you will need to contact Apple - try a chat at the online Apple store to see if you can get an explanation.