are you activating new line or replacing sim card on your exitsting account?
For activation, eaiset is to use app to activate. If you use the Web, you can click Subsribe online when you see the plan
if replace sim card existing account, you will need to login My Account and go to profile to change sim card
The website is finicky. Try a different web browser, clear cache or incognito mode. You can try a different device (PC or labtop).
You can also download App to activate account. The $15 and $25 plan is only available for activation via the App. The rest of the plans can be activated via browser or App.