I agree with Scott in that you need to post at least a screen capture if not a copy of your plan, to not do so makes us have to guess and there are LOTS of wrong guesses based on sparse data from you. (Just like you, we like to be right for you and ourselves)
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I would open the space's Room Specification Dialog - Structure Tab and make sure that the "Floor Finish" is not set to "Zero inches" in thickness ( a common cause of "no floor", Zero inches thickness = no visible floor)
When anything is a miss, the first thing you should get interested in is Dialog Box Settings, find out what they are for by way of study and practice; this software runs or fails to run based upon Default Settings and dialog box settings. There are countless numbers of them to find out about....Start!
DJP
