I have a problem with a paypal sandbox test buy

I am doing e-commerce on shopify.And I finished mine website.And trying add payment methods.Finally I intergrated Paypal express check-out on shopify.

And I wanna test paypal its working correctly. And I create sandbox account.I can login smoothly www.sandbox.paypal.com but when I am trying test buy on my website paypal login says "Check your email adress and password and try again" but I am sure with mail adress and password are corrects.