By default, ThirtyBees does not have the ability to manage product quantities with decimal parts.
There are modules sold for Prestashop that supposedly add such functionality, but these modules use hundreds of core method overrides and only introduce a huge of errors into the store engine.
Therefore, a better solution is to switch to smaller units of measurement in product quantities. For example, instead of a kilogram, it is a decagram or gram, instead of a liter, it is a milliliter, etc.