I tried your instructions in previous post. Basically I created a structure group, wrote a script for that and created some properties:

I created thickness1, thickness 2 and thickness3 for span1,span2 and span3, respectively. And then I wrote this script:

Then I went to the optimization tab and adjusted it like this: 

I chosed parameters that I created before, and set a frequency domain field-profile monitor which is named "monitor_Ref". Objects tree looks like this: 

So far it worked well. It creates a random structure and saves files to analyze. But after finishing the first generation of simulations, it gaves this error:

I tried to fit the monitor into the simulation region, changed the type of monitor to frequency domain field and power but none of them worked. Since this is the first time I try to use optimization tool so maybe I'm missing something, even though those are pretty straight forward. What could be the reason for this issue and how can I solve it?