The main problem seems to be that there is no toilet flange, i.e. the toilet was just set on top of the closet bend (that’s a 90 degree 3′ or 4″ waste elbow designed especially for toilets). Even if the bend comes to a good height at the slab and even if a good wax ring with an ample sleeve is used, such an arrangement will leak sooner or later. Believe it or not, as stupid as this plumbing arrangement is, I’ve seen it dozen’s of times in my repair work.
You will need to install a flange on the closet bend at the correct height to connect to the toilet. The flange must lay flat just on top of the slab or floor. The easiest flanges to use are those that have a rubber gasket that slips into the bend and is tightened by srewing down the flange. These are made of ABS plastic or cast iron. I prefer cast-iron, especially on a slab. You may need to trim the closet bend. There are special cutters for this purpose. My favorite is a Rigid Inside Pipe Cutter. Then, attach the bowl to the flange with the standard closet bolts. Don’t forget the wax seal.