I just went through something similar:
https://xoutpost.com/bmw-sav-forums/...ncomplete.html
Mine was the O2 sensor *heater* readiness monitor being incomplete. Is that what yours is? I believe that is the one that takes the longest to complete under normal driving, even when everything is perfect.
I did replace the rear, original O2 sensors (and had replaced the front ones 30k miles earlier), and then waited about 200 miles of regular driving for the monitor to show as ready. But I am not sure at all that I needed to replace the sensors. I tested resistance of the heater elements in those sensors and they were normal, but I ran out of options and patience and did the replacement.
You should be able to check those readiness monitors on your OBD2 reader, unless it is a really low-end model. So that would at least take any mystery out of the smog check. No need to go there again until your reader is showing no codes and all monitors ready. And once you've got that, you should be sure to pass unless you fail the visual inspection.