Microsoft store problem

I am having a problem trying to purchase a game in Microsoft Store and am trying to get some help. I log on to Microsoft Store, find the game I want, click on the grey "buy now" tab and a pop up box appears for my email address. i enter my email address and another box pops up asking for my security number. i put my security number i then a box pops up saying you've signed out" and the screen returns to the initial store home screen.

Any help or advice would be gratefully received.