I have found the current offer T&C’s and they should have started accepting claims on the 10th April 2025.
… redeeming the cashback reward between 00:01 (local time) 10th of April 2025 to 23:59 (local time) 16th of May 2025 (both dates being inclusive) using the Cashback Claims Portal.
Terms and Conditions
Support are aware that there has been a problem, and have solved it for at least one customer.
Presumably it is being fixed so you may want to keep trying again:
See: