Can you put a hot tub on grass? YES, but you cannot put a hot tub on the grass directly. You will need strong hot tub base on grass to support. It should be on a stone or hard pad, never directly on the grass. The dew and moisture from the ground could get into the workings and ruin and rust them. My recommendation instead of having concrete set is to get a bunch of those pavers and use that as a base over grass.
The weight is one problem that most hot tubs have in common. The site has to be able to hold the weight of the tub, the water, and the people in it no matter where you choose to position your hot tub. The base for your hot tub must be strong, and it also has to be able to support all the necessary hot tub electrical and plumbing installations.
Not only will the moisture ruin the hot tub, but the hot tub would sink into the ground. It expands the weight and slightly hardens the ground when you put on a hard surface. Also if it is not on a platform, one side will sink into the ground further than the other making it unleveled. I would also recommend digging down until the spot is hard enough.
I suggest using prefabricated hot tub Base if you want to put a hot tub on grass. There is a system of interlocking hot tub pads that can be used to build a base on just about any surface including grass. These are declared to be as efficient as poured concrete but a lot cheaper.
You can use a prefabricated hot tub foundation pad that will hold over a spot in your backyard and keep your spa level. You will still need to make the area of grass and topsoil, but laying a hot tub pad is an effortless way to get a luxurious-looking strong base on grass.