What base preparation do I need to do for a plastic shed: can I not just set it on my grass?

No, unfortunately you cannot just place a shed on grass. All sheds need a strong, level base such as concrete, tarmac or paving slabs. For true stability, you will need to use hilti screws to attach your Palram Canopia shed to the base. (Hilti screws are not included but are available at DIY stores).