I hate to post easy questions on the forums, but there are not any search results for this one.

I am in the last tutorial and placeing a trade depot.  I get the red ring around my desert planet when trying to place it, and no matter where I go, the red ring stays and says "another structure is blocking..."  I am putting the structure where there is nothing else; however, and it's a bit frustrating.  What's the secret?  I tried every inch surrounding this planet, within the lines and not near anythign else, yet I cannot build.

Thanks