You may not be able to enter your phone number for the following reasons:
1) You may have input an invalid phone number format (type in your number without country dial code of +65). If your mobile number is +6592345678, you would just need to enter 92345678)
2) The phone number might have registered under a different ShopBack account.
For assistance, please contact us at email@example.com or live chat agents here and we would be glad to assist you further.