Do I need foundation for my Greenhouse?

I'm planing to build up a greenhouse. I alread purchased the palram kit. I'm mainly focusing on cucumber and flower planting. I currently have 19*18 ft space in my garden.
According to site https://avastips.com/greenhouse/greenhouse-gardening-preparing-the-site/
they suggest to build a perimeter foundation or have it on a concrete raft. Can I just build my greenhouse directly on the ground?