During the height of the pandemic, most of the intercity rail operators in the UK introduced compulsory reservations, at least on paper. In practice these were only enforced at a few locations even at the height of the pandemic.
Some operators removed the compulsory reservation flag from the timetable data but Avanti and LNER are still setting it despite all reports being that they are no longer enforcing reservations (and indeed at Liverpool Lime Street the station is not set up for enforcing reservations).
The pandemic and industrial relations issues have also made a mess of timetable planing. Particularly at weekends, this is causing reservations to be released much later than they should be. The retail sites do not* distinguish between a train for which no reservations have been released yet, and a train for which they have sold out.
Unfortunately websites nearly always insist on a valid itinerary before they will sell you a ticket. If the timetable data says that the train is reservations compulsory and the reservation system cannot provide a reservation they will not sell a ticket against that itinerary.
If you really want to buy a ticket before reservations are released there are a few possible workarounds.
- If buying a return ticket that is valid for a month on the return, select a different date for the return. This won't help you for this particular journey since you have already bought your outward ticket, but it's something to remember for the future.
- You can use operator and/or route options to force an itinerary that only uses "regional" trains that do not have the compulsory reservations flag set. Trainsplit/trainscanbecheaper are particularly good for this as you can allow/deny individual operators compared to most sites which only give the options of "any operator" or "one specific operator". Note that if you force an itinerary that only uses West Midlands Trains (aka London Northwestern) the website will probably try to sell you an operator-specific ticket, it should be possible to select the "any permitted" ticket instead though.
- There is apparently a loophole on the southern railway website, you can plan the journey for a different day, select the ticket you want and add it to your favorites. Then from your favorites you can buy the ticket for whatever day you want without an itinerary.
But ultimately there is little point, you won't be able to buy advance tickets for a train until reservations are released for that train and if you want a flexible ticket you should be able to just buy it on the day.
* Until recently they could not, I think a recent upgrade to the backend systems means they now theoretically can but I don't know of any that actually do.