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.

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

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?

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.