cannot log into iTunes

recently purchased a Mac. trying to log into my attends account. it won't let me. constantly sends a message to my iPhone for verification. says I need to change my password. I do. then when i go back to the mac and try it, it says I don't have the right password. This has happened 5 times now. I have changed the password in my iCloud. I have tried shuitting down my computer. but everytime i go back to log in the same thing happens. its doing my head in. any help appreciated.