function[mmm_?NumericQ, nnn_]

I tried to define your function in NMinimize and it runs without error message. However, the function is stochastic in nature. I doubt this type of function is suitable for NMinimize.

First of all, thank you alot.

Could you upload that file? And I know the issue with the Randomness but I assumed if I do it mutliple times there, it is a start (I had no better idea at the moment)

I do not know what is getting in the way. Here is a simple case that works as I would expect.

function[x_?NumericQ, y_?NumericQ, z_?NumericQ] :=  x + y^3 + z^6

 NMinimize[{(function[x, 1, 2] - 70)^2, 3 < x < 4}, {x, 3, 4}]

(* Out[10]= {1., {x -> 4.}} *)

It may be too simple though, since it would also work without using the ?NumericQ pattern predicate checks.

Might try defining function a bit differently:

function[x_?NumericQ, 1, 2] := ...

See if that improves matters. if not, more detail will be needed in the question.