Why does the system not allow me to register?

If you have previously registered an account with your current mobile, you will not be able to use it to register a new account. This is because every mobile number can only be used once.
This is also the case with Student/staff/resident ID's - if you have previously registered an account with your ID - you will not be able to use it to register a new account. If you switched your mobile number - you can simply update your existing account with your new mobile - there is no need to create a new account.