How to set the unit of prepomax to mm, kg, s, c? The default option does not have this combination.
Apart from the 4 predefined sets of units, you are left with Unitless where you can use any set of consistent units. They just won’t be written anywhere (like in Abaqus or some other FEA software).
But, as I said, the units have to be consistent. If you check the derived units from the base ones you specified (you can do it manually or here), you will notice that they aren’t very intuitive.
Only a consistent unit system can be used. But If you choose ton for mass as a unit system, you can later enter mass in kg in all entry boxes. The entry boxes convert entered units into the unit system’s units.