it is definitely possible to use only one.
If so, call the preop anat_*.nii (e.g. anat_t1.nii or anat_t2.nii or anat_pd.nii) and the postoperative one postop_tra.nii (no matter if it’s transversal, coronar or whatelse. The postop_tra.nii is the only needed file and if there is only one, one should call it that way.
It could well be that the status bar is outdated – so never mind what it says. I never pay attention to it much so it could be that the code that controls it is not correct anymore.