Recommended foundation or base
Your hot tub needs a solid, flat, level foundation. The space it sits on must be able to support the weight of the hot tub, the water in it, and those who use it.
If the foundation is inadequate, it may shift or settle after the hot tub is in place, causing stress to the shell and cabinet, voiding the warranty.
Your Spa will weigh anything between 1200kg and 2500kg (that's between 1.2 and 2.5 tonnes, the same weight as an average family car or 4x4)

Concrete Pad (100mm / 4" or thicker with reinforcing mesh) |

Timber Decking (with concrete foundations) |

Existing Paving (on concrete base) |
Also consider the surrounding surfaces - Block paving is often laid with sand which can be a problem as the sand is trodden into the spa water and can damage internal components. Decking is a good clean surface to use, but can become slippery when wet, and gravel can be uncomfortable on bare feet.
By minimising dirt entering the hot tub on bathers' feet, your water will look better and last longer.