C&B isn't SE's system so they can't "fix it". The error you're experiencing is due to C&B not being available for your country, as the email says, because if I recall, Singapore falls into the SEA realm of billing options, which isn't C&B.

Just wait till january to do it as that's when new options will be available to switch to, everyone will need to reactivate then regardless so payment options doesn't matter now.