Why does this say I already have a user account for my phone number?

When you activate a prepaid sim card, it automatically creates a pay go online account for you with your 10-digit phone number as your user ID. You would use the alphanumeric password you created and the PIN code that you created during the activation process.

You should continue to use the pay go online website to manage your prepaid service.

Prepaid does not make an account. The account is created automatically when the SIM card is activated. You are likely trying to login at the wrong place. To access a prepaid account, go here:

https://www.paygonline.com/websc/loginPage.html