us2mq8j1myp4 I did some testing and haven't run into this at all. So, the only thing I can think of is the game is still seeing the lot as a normal residential lot. So, first make sure that you have set the venue to residential rental. Then try to see if you can assign the rooms. If that doesn't work, then save the lot and exit. Exit the game and delete the localthumbcache. Then restart and go back to the lot in build from the map screen and see if the venue is still set to residential rental. Then try to assign the rooms.
Hope this helps.