If you have an existing offer on your account for the same membership then you can't apply it.
Try speaking to staff via live chat. Method in my signature below.
I’ve not got entertainment they sent me an offer today 4th march for 3 months at £4.99 , but it says in the small writing use this offer before 22/02/22 as I can’t go back in time it’s impossible to use
Try send an email to customer services, that way you can't get cut off 😁
Trust me I know your pain, I used live chat a few weeks ago and got disconnected a few times, it was a testing time!