Had a second look at your sheet and of course Richard is right, you hadn't setup any iteration at all, just a bunch of equations.
You had an error in the volume equation - the whole expression has to be multiplied by 1/3, not just the first summand.
The problem is underdetermined. You can freely chose 2 of the variables (within certain ranges) and calculate all the others - e.g. you can make all values dependent on x1 and y1, or on p2 and p4 or, as I have done in the attached sheet, of t and S.
So also the volume is dependent on just those two variables. That means you can freely chose one of the two, lets say t (for the allowed range see the comments in the sheet) and calculate the other (S) so that the object has the demanded volume. You can look at the volume as a surface in 3D:
Mathcad's 3D plot doesn't allow us to suppress unwanted or invalid values, so in the above plot I have set all values corresponding to invalid combinations of t and S to zero. Thas the reason for that scarpe face.
So you can see that your problem has an infinite number of solutions (the red plane represents Volume=N).
You could add an additional constraint (like S=t or anything else) to get a unique solution.
As long as we don't do that we can also plot the solution curve (S over t) in a 2D plot
BTW, what is shown here is the complete solution curve - there is nothing more to the left or to the right of it!
You can also plot the other values, e.g. see p1 here
and as you can see it weill never go up to 260, as you wrote in your initial post - at least not with the values for L, B, Mx etc. you provided.