You have exactly *one* instruction page in /postinstructions
<expt>
/ preinstructions = (instruction_IT)
/ blocks = [1= IT_ZT]
/ postinstructions = (test_end)
</expt>so there is nothing to go back to. At this point the block is already over.
If you want to be able to navigate back and forth between your "survey" pages and your "instruction" pages, you should not use <htmlpage> or <page> elements via /pre- and /postinstructions. Instead set up additional <surveypage>s to display your instructions and integrate them with the rest of your survey block.