You should be able to choose your cabin, but you need to search for the choose cabin button, which then takes you through a selection process for the location and then offers you a selection of available cabins, or it may pre select one for you, then you need to change it to a preferred one if you have one. Its very fiddly and time consuming, can lead to errors which you then need to correct.
Which is one of the reasons I use a good online TA who does all this work for you.