@mountainsunsets
And as far as I understand it, that means OP should have contacted them within that 30 days and given the retailer a chance to solve the issue - eg. giving her a refund or a replacement. If the refund or replacement never arrived, then she could take it further.
I find it very hard to believe that you can just go to a company months after placing an order (and after no attempts at communication) and get a full refund.
You really are not understanding it. Citizens' Advice are saying DON'T contact the retailer before 30 days asking for a refund or replacement because the amount of time considered reasonable by statute for them to get the item to you is 30 days. Of course you can contact them to start looking for the item sooner, but they don't have to entertain a request to refund or replace until 30 days have elapsed. There is absolutely no statutory maximum time under 6 years within which you must notify or make a claim.
However, as I said above, it is possible for a retailer to insert into their standard terms and conditions a provision that requires all non-delivery to be notified within a particular timescale. We do not know if OP's contract contained such a clause. If it did, the seller can rely on that as a matter of contract. However the term would have to be brought to the consumer's attention at the time of purchase and cannot be unreasonably short otherwise it would breach consumer law.
Well-advised retailers do this (often at the request of their insurers) to protect themselves in situations where too much time has elapsed for them to track down the missing package. They do it precisely because the law does not give them this right by default!
As to the point that people keep coming back to -how can OP prove that she didn't receive the package? She swears a witness statement saying "I did not receive the package". This is her evidence. It is open to the other side to attack her credibility, and witness evidence alone is never as good as witness plus documentary evidence, but that's all she has to do. In the unlikely event that the case went to trial the judge would listen to her and take a view as to whether, on the balance of probability, she was telling the truth. It's simply not correct that she has to start off by proving that she is not lying.