You can always just buy a Sail Rail ticket on the day from the ticket office at your local station (or Guard onboard the train if there is no ticket office) so this is an option if you can not buy online. These ones are only a couple of pounds extra (i think about five or six pounds more depending on your journey) compared to the advance tickets. These ones do not sell out. I have always done this so this is certainly an option if you are unable to buy online.
This is stunningly bad advice.
The tickets, including the £5 more expensive Sailrail standby/SailRail Single are set to only be issued with a valid ship reservation. If these are not available in the system for whatever reason you will not be able to get the ticket issued.
I know this, I have tried and failed.
Obviously as there are no tickets on any day available the ferry "reservations" haven't been loaded, the only resolution for this is to contact TfW Rail and try to get someone responsible to fix that.
BTW these ship reservations have nothing directly to do with the ferry companies, they exist entirely on the GB rail reservation system and are not in any way connected to the ferry companies booking systems.
The ferry operators are not at all interested in the reservations at check-in, only the actual SailRail ticket is checked.