sandbox not working

Hi
I have a developer account and setup a test business account and test buyer account – when i select buy using shortcode [wpecpp name='Personalised Christmas theme Youtube Video' price='5.00'] the shortcode opens paypal with the correct purchase details but when i try yo login with my newly created buyer account paypal asks me to check my user and password…

help

https://ww.wp.xz.cn/plugins/wp-ecommerce-paypal/