Same here. Originally thought I was because the TiVo box I was trying to watch on was a new addition but tried the old one (had for over 2 years) and it says the same 'upgrade to watch' also it's on Stella and all the strike back boxsets
OnDemand shows broadly fall into two categories - "CatchUp" which is immediately after broadast and generally for 30days, available inclusive to all packages. Then "Boxsets" which is most other stuff - so old series, reruns etc, and is only available to "Full House" or "Maxit" customers.
Without having checked any of these, which TV package are you on?
I have maxit tv so should be able to watch sky one on demand. I spoke to customer services about this on Tuesday and was told they were aware of the problem and it would be sorted out shortly. That was 4 days ago and still can’t watch programmes I’m paying for.