when i try to create a new mailbox in iphone the message is "The mailbox couldn't be created on the server" Now what?

That sounds like your email account is configured as a POP account, not an IMAP account. Go to Settings>Mail>Accounts>tap on the account you are trying to add the new mailbox to. On the next page, what is the heading above the account? Does it say IMAP or POP?

That is why you cannot add a folder on the server. You may be able to add a local folder on the iPhone itself, but that will only exist on the iPhone, since it won't sync. I would test that for you, as I do many of the problems users have that i have not experienced, but I do not have a POP account to test with, sorry.