Usually, it's one of two reasons. The first is that we may not be able to ship to your state. Federal and state regulations restrict the states and counties where we can ship to. Check out that list here.
The second may be as simple as using a different credit card. Refresh the page or delete the cookies and cache from your browser, which may be blocking the new credit card information from going through. Please contact us if you need further assistance.
For billing issues, we recommend you live chat us via the Support button on our site. Our hours are Mon-Fri 6am-5pm PT or Sat-Sun 7am-4pm PT. (Please note that our phone support line is temporarily unavailable due to the pandemic.)