For which solver? This is a 1-variable problem. If you want help, it does help if you explain more than you have done.
Note that ~= is not something you can specify for ANY solver, since that allows your solution to be infinitely close to equality, yet not exactly equal. And that makes your problem ill-posed.
Anyway, is this problem even meaningful?
err = @(X) (abs((((F)*(X/((F).*(1/M-1)+X))+(1-(X./((F)*(1/M-1)+X))).*X)-D)/D))*100;
It looks like I missed a dot in there to vectorize it. Regardless, there seems to be only a single minimizer, at X == F. We can probably prove that to be the case with a little effort, merely by showing the behavior of this simple function as X grows large in both directions, that there are no points where the function has a zero derivative.
We could play with that and now look at the derivatives.
Or, is F also an unknown? If it is, then your objective is not formulated in a form that any solver in MATLAB will directly take. You seem to be treating F as a constant.