If the septic tank is no longer in use and has been filled in then you can usually build over the old septic tank without removing the septic tank first.
However if the septic tank is still in use then you cannot build over the septic tank because you need access to the septic tank lid to clean it out.
I do remember though a guy who built a garage over his septic tank but he lives rural and he kept the lid to the septic tank accessible.
So it was easy for the the septic tank to still be cleaned out and he just had to move is his truck out of the garage to access it.
His plot of land was small and there was no where else to build the garage so he built it over the septic tank and it's been okay for around 20 years now.
But most places will not allow you to build over a septic tank so you would have to check with your local building codes.