If the septic tank is active, it's a bad idea to build anything over it because the tank will need to eventually be accessed to be pumped. Secondly, if the field was to ever fail - which eventually they do - the whole area will need to be dug out. Whatever route you take, you need to make sure to provide access to the tank, and be aware you might need to replace the field altogether - which would presumably involve replacing a patio or deck built on or near it, which you probably don't want to do.
If the septic tank is inactive, though - if it was from a system no longer used, or has been converted to city sewer - then the tank needs to be properly abandoned so nothing can collapse in it. Once that's complete you can build over or around it. Good luck!