You may not be able to update your phone number due to the following reasons:
1) You may have input an invalid phone number format (type in your number without the country code +66. e.g: If your mobile number is +66991234567 you would just need to enter 1234567)
2) There may already have been another ShopBack account registered under your phone number
For assistance, do reach out to our friendly live chat agents here!