Have you marked the box agreeing to the terms and conditions? 

If you are still unable to complete your checkout it may be due to the cookies that are stored in your browser.

You can do a search and find how to clear your cookies and/or use a different browser.

You will only be required to set up your account once.

When you have future purchases the system will recognize you once you have entered your email and ask you to log in to complete your purchase.