Hi @mollieeryan thanks for posting and welcome to our community.
Sorry to hear you're having difficulties registering your online account. Firstly is your SIM, O2 or Virgin Mobile? What type of online account are you attempting to register with us? Is it My Virgin Media? Is it for Virgin Mobile? Also, do you know if your mobile account has been migrated over to O2? If so then any online account for mobile would need to be set up here. Please do get back to us.