Whichever foundation you choose for the greenhouse it must be level and horizontal. Some greenhouses can be erected direct onto earth.
They are pinned to the ground using anchors sunk into concrete. But the choice of foundation really depends upon the level of humidity that you wish to have in the greenhouse and how exposed the site is. A concrete slab base will provide quite a dry, arid atmosphere whereas earth gives a higher level of humidity. The options include: a concrete slab, a concrete "picture frame" for the greenhouse to sit on with earth inside, paving slabs, earth, a brick foundation or a dwarf wall.