I suppose that's SNCF making their customers’ life difficult. Nothing new unfortunately.

These TERs can't sell out.

Edit: there are engineering works on that line. The timetables shown seem correct and are TER replacement buses. There may be a problem with the module that looks up the fares.

This is what I’m seeing for all times and dates:

So does this mean, you can’t reserve and you just pitch up and buy a ticket or does this mean the line is full and/or not running?

The app incorrectly claims for many, if not all, TER buses that reservations are mandatory. However, that is nonsense.

What do you mean by booking ? Reservations are not possible on buses, simply log the bus to the app like a train and hop on.

Your pass is valid. If the driver is confused tell them it is a valid SNCF ticket.