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 help@shopback.sg or live chat agents here and we would be glad to assist you further.