Thanks for your post. I'm sorry to hear you're having difficulty managing your GoPhone account online. Was your number on a post-paid plan previously? If so, the system may still recognize it as such, which is why it may be giving you that error.
It's difficult to say for sure, though, so I recommend you send us a message by clicking here so we can go through the process with you step by step. Please include your name, phone number, email address, and the best time to reach you. You can expect a reply within two business days, so keep an eye on the little blue envelope icon in the top right corner of your screen.
In the meantime, please let me know if you have any additional questions or concerns!
*I am an AT&T employee and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.